Toei Studios is an animation studio based in Japan. They are responsible for most of the animation produced for the Generation One television series. They are known for animating many American and Japanese cartoons, such as G.I. Joe, Devilman, Sailor Moon, Voltron, Tranzor Z, Dragonball Z, One Piece, Digimon, Real Ghostbusters, Zatch Bell, Bobobo-bo-Bobobo, the original Teenage Mutant Ninja Turtles, Spider-Man and his Amazing Friends, Pryde of the X-Men, Kinnikuman...look, it might just be shorter to list the stuff they haven't worked on.
Toei Studios animated the entirety of season 1 of the The Transformers cartoon, including the three-part pilot episode More Than Meets the Eye. Following that, season 2 got split up between Toei, AKOM and an unnamed studio from the Philippines, with Toei animating 39 episodes (the highlights in terms of quality being "Atlantis, Arise!" and "Auto Berserk"), whereas the other two studios would handle the remaining ten episodes. Toei also animated Transformers: the Movie and the Scramble City OVA (which contained 15 minutes of all-new footage) as well as all the animated television commercials and all the season 1 and season 2 bumpers. Season 3 (Transformers 2010 in Japan) saw a decline in Toei's participation, as they were gradually outsourced by AKOM's more cost-efficient animation, thus only producing 13 episodes for this season. The short-lived Season 4 (the three-part "The Rebirth" series finale) was then handled entirely by AKOM.
Additionally, during the entirety of the Generation One line, Toei would be solely responsible for the unique animation seen in all Transformers toy commercials, even during and after 1986, when the animated series itself had largely switched to AKOM.
After Hasbro stopped funding further Transformers cartoon episodes, Takara would retain Toei to animate all of the Japanese-exclusive cartoons: Headmasters (35 episodes using all-new footage), Masterforce (43 episodes), Victory (38 episodes) and the Zone OVA.