Grimlock is an Autobot with the role of Lieutenant Commander who transforms between his robot mode and his dinosaur mode.  He belongs to the group called Pretenders because he (as well as the other Autobot Pretenders) disguises his robot form inside a shell resembling an Earth being, in this case a human. 

Grimlock is the most vicious and powerful of the Dinobots.  The fact that his Pretender shell resembles a human makes him extremely upset and angry.

The Generation 1 Grimlock was released 1989.