Super Sentai, since Battle Fever J, has the climax of most episodes being a giant fight between an enlarged monster and a big robot piloted by the heroes, with of course, the monster being defeated. In early sentai shows, those robots were machines, which were, starting with Sun Vulcan, built from the combination of different mechas, which together, made the giant robot. From Battle Fever through Jetman, the mechas and the robots used to fight the giant monsters were machines. However, with Zyuranger, which made Super Sentai switch from a mostly sci-fi genre to a more fantasy like tone, for the first time, the mechas were sentient beings and the robots were much more than mere machines.
Since then, several series had living mechas which were living beings. Here is a little history of those living mechas
1) Sugimura era (Zyuranger, Dairanger, Kakuranger) : the era of the gods.
Zyuranger was one of the first sentai series where the mechas had an animal theme (Liveman was the first, but the mechas were machines), in that case, ancient animals (notably dinosaurs). As the heroes were ancients warriors who were living during the dinosaur era and belonged to five different tribes, each ones of their mechas were in fact the Guardian Beast of their respective tribes (Red/ Tyrannosaurus, Black/ Mammoth, Blue/ Triceratops, Yellow/Smilodon, Pink/ Pteranodon). Since Red's mecha, the Tyrannosaurus was able to fight on its own and appeared earlier, while the others were mostly able to be involved while combining, Red/ Tyranno had the most focus among the five main ones. However, Daizyujin, the robot coming from the combination of the five Guardian Beasts was shown to be a God, and as such, the five Beasts were actually together the God of the Zyurangers, and were able to communicate to them as a supreme entity, who had power and wisdom. Daizyujin was able to talk and had a lot of influence on the Zyuranger team. Dragonranger's mecha, Dragon Ceasar, was more written as the partner of Dragonranger, and in the episode following Burai's death, his grief has been highlighted, and he really was written like a character with feelings. Because he was able to fight on its own, Dragon Ceasar had a lot of focus, together with Burai/ Dragonranger. The last mecha, King Brachion, was also shown as a living entity. Zyuranger was the first show to introduce sentient mechas, and both Daizyujin and Dragon Ceasar showed two different ways to write the relationship between the rangers and a sentient mecha helper.
Dairanger followed the trend of Zyuranger by having the five Mythical Ki Beasts which were the Guardians of the Dai tribe. Like in Zyuranger, the Red ranger's mecha, Ryuseioh, a dragon mecha, had more focus, since it was introduced earlier and could fight on its own by changing into a robot form. The four others (Green/ Shishi(Lion); Blue/Tenma (flying horse), Yellow/ Kirin and Pink/ Houhou (phoenix) were written in a similar way as their Zyuranger counterparts. However, unlike Daizyujin, Dairenoh wasn't a god-like entity. Kibaranger's mecha, was a tiger-like mecha who could transform into a robot form. However, Kibaranger's most prominent partner wasn't his mecha, but his white tiger themed talking sword. Lastly, the final mecha of Dairanger was very intriguing, because it had an human form, and was able to interact with the other Dairangers, especially Kazu like a fellow human. While its real form was one of a giant tortoise, Daimugen was able to become a little tortoise as well as an human. He was the first mecha with whom the Dairanger could interact in daily life as a friend.
Kakuranger put more focus on the "mechas being gods" concept. Indeed, the two main robots (Muteki Shogun, and Kakure Dai Shogun) and an additional hawk like mecha (Tsubasamaru) were in fact, gods which together were called Sanshinshou (Three God Generals). They were shown as god entities, similar to Daizyujin. Another character, Ninjaman, was introduced as a flawed pupil of Sanshinshou, and was both a "sixth ranger like" character, being an additional warrior in human size, while being also able to grow big and be involved in giant fight like the God- like mechas.
After those three series, Super Sentai switched back to machines like mechas. While Ohranger had Gunmajin, and Carranger had the two wild Cars (Pegasus and Dragon) being sentient creatures, the trend of having mechas being living partners only came back in full force in Gingaman
2) Gingaman and Gaoranger : the animal partners
While the Sugimura era had god like mechas, Gingaman was the first series having each ranger have a true mecha partner, that partner being a giant beast with which he/ she could communicate more easily. In Gingaman, each ranger has a Seijuu, a giant beast coming from the stars, as a partner. Three years later, Gaoranger used again that concept, the rangers's partners being the Power Animals, which were also giant beasts.
In Gingaman, the relationship between the Seijuu and the rangers was a lot highlighted in early episodes, when the heroes were fighting the monsters with each Seijuu attacking separately. Indeed, Gingaioh, the giant robot coming from the combination of the five Seijuu only appeared in episode 7. Episode 9 also allowed Gingat, Saya/ Gingapink's partner to have some additional focus. While the Seijuu were unable to talk, the Gingaman could talk to them and heroes and beasts were able to understand each other. With the appearance of Kurokishi, another mecha, Gotaurus appeared, and the relationship between Gotaurus and Kurokishi was a key element of the show, going on as Hyuuga became the new Kurokishi. Indeed, Bucrates was able to force Hyuuga to work for him because he was had Gotaurus in his power. Three other Seijuu, Gingarhinos, Gingaphoenix and Gingabitus appeared later. Gingarhinos and Phoenix were pretty heartbreaking cases, where the Seijuu were only spirits being able to live in a completely modified body.
Gaoranger used again the concept of the animal mecha partner, in a similar way. One big difference between Gingaman and Gaoranger was that each hero had more than one partner, and a big part of the show was focused on having the heroes discovering new partners. Like in Gingaman, the Gaorangers were able to talk with their partners and while the Power Animals couldn't talk, they still could understand each other. While not all the Power animals were able to get a lot of focus, some of them were more highlighted, like Gaolion and Gaobison (notably in arcs when they wer einjured) among the main mechas, Gaobear, Gaopolar (especially in their intro episodes where they had an human form) and Gaodias among additional ones. In the Rouki arc, the rangers were very affected when they lost contact with their mecha partners (for example White losing Gaoelephant). One device with which the heroes could have a contact with their Power animals partners were the Gaojewels. Lastly, the Power Animals were living in a floating island, the Animarium, where the Gaorangers were able to visit them.
Gingaman and Gaoranger were the two main series introducing the concept of the ranger having independant animal mecha partners
3) Abaranger and Go-onger : the era of the talking mechas
Two years after Gaoranger, Abaranger brought back the concept of animal mecha partners, using again the dinosaur theme. However, Abaranger introduced a new concept, with each mecha (the Bakuryuus) being able to speak Japanese, and as such, having a much more detailed interaction with the heroes. Each ranger had an individual partner, with whom he could talk and interact. As a result, the relationship between heroes and his/ her Bakuryuu partner was much more fleshed, since the Bakuryuu could show very different personalities (the hot blooded Tyranno, the shy young Triceratops, the wise Brachiosaur, the cynical Tupuxuara (Topugeira). Besides, the Bakuryuu could appear in much more scenes than the giant fights, interacting with the heroes either through their henshin devices, or appearing ot them in a big screen in their base. Heroes and Bakuryuu were interacting very freely, and because they were able to talk the same way, were interacting as equals.
Go-onger used the same concept with the Engines, with a little difference : the Engines were mostly shown as little models, and were interacting with the heroes as holographic entities coming form their phone like devices. The Engines were only able to come big during a short amount of time during the giant fights, and most of the time, stayed in miniature size. However, the relationship between the heroes and their Engines partners were similar to the one seen in Abaranger between heroes and Bakuryuu, a relationship between equals.
4) After Go-onger : different concepts
In Shinkenger, the Origami were shown to be sentient entities, but the relationship between the heroes and the Origami wasn't hightlighted, unlike what was seen in Gingaman anf Gaoranger. Shinkengold however had a robot partner, Daigoyo, a lantern like entity that could talk to him and also become a big robot when necessary. The Gosei Headders were similar to the Origami as sentient entities, but the relationship between Headders and the Goseiger was never explored. However, one of the Headders, the Groundion Headder, was able to get more autonomy and became one of the first non human sentai hero, Goseiknight. Brajira's final MOTW were also Headders.
Gokaiger, as an homage series, brought back the concepts of previous sentai series, with Gaolion returning, and a new Engine, Machalcon, appearing, each mecha having a similar concept as those fomr the series they were tributing ( respectively Gaoranger and Go-onger)
Go-Busters was an interesting case: each hero had a robotic partner, the Buddyroids. The Buddyroids were full characters on their own, and since they were human sized, were able to interact with the heroes in a daily basis, and were written as major characters. Cheeda Nick, Red's partner, was alos being able to become the hero's bike. Another Buddyroid , Beet J Stag, became a Go-Buster himself, the silvery Stag Buster. While they weren't "talking mechas" strictly speaking, since the mechas were in fact machines, the Buddyroids of the three main heroes were a major part of the mechas, since they were necessary to have the mechas being able to move. Beet J Stag was the pilot of its own mecha, as a ranger itself. The three main Buddyroids were also able to become the armors of the heroes when they were using theur Super Mode. Of all sentai series, Go-Bsters was likely the one where the relationship between the heroes and their robotic partners was the most detailed and explored, with sometimes very heartbreaking situations, like the one between Beet Buster and his partner Stag Buster.
Lastly, Kyoryuger both brought back the concept of the dinosaur mecha, and the concept of the partner mecha as seen in Gingaman and Gaoranger. However, so far, Gabutyra had by far the most interaction with his ranger partner, first as the first mecha to appear and fight enemies, then by being able to become smaller and become Kyoryured's additional armor with the Carnival Battery. Unfortunately, the relationship between the other rangers and their mechas partners has been very little explored so far, with Plesion having so far the most focus (Pteradenoh had also some focus, but it was because it was brainwashed by the enemies).