The Dinobots were created for the Autobots with the authorisation of Optimus Prime. Initially they were of low intelligence, but this was upgraded later to make them of use to the Autobots.
Grimlock is extremely powerful, especially when he transforms into his Tyrannosaurus Rex mode. In particular his powerful jaws can destroy amost anything. In some versions of the Transformers universe he can even breath fire, making him more akin to a dragon than a dinosaur.
Grimlock is a brave and loyal fighter on the battlefield, however his loyalty is not always maintained away from combat. Grimlock is highly ambitious, you could even say power-hungry. He believes himself to be superior to Optimus Prime and thinks that he should be the Autobot leader. This desire for power and refusal to be subordinate has led him into trouble more than once, leading ultimately to him leading the Dinobots to leave the Ark and Optimus Prime's rule.
Although Grimlock is no genius, it's easy to underestimate his intelligence due to his primitive speech mannerisms. In fact he did at one point become a genius, in the popular episode Grimlock's New Brain which first aired in 1986/1987. An accidental dose of anti-electrons boosted his intelligence and in this form he created the Technobots.
A giant robot that can transform into the most fearsome dinosaur - perfect! Needless to say, Grimlock is a very popular Autobot and are the various toys and action figures of him you can buy. There is speculation in the Transformers fan community that Takara will soon release a Grimlock model in their Masterpiece range.