The Dinobots were added to the Autobot Transformers toy line in 1985, the same year that the Constructicons and Insecticons were added to the Decepticon team. The toys were actually licensed to Takara from another toy company which used them as mechas for human pilots, explaining why they all have tiny compartments inside them. (The Insecticons have the same compartments.)
Young boys, when taken as a group, have always had two main loves: giant robots and dinosaurs. Putting them together in the same toy, along with some simply excellent design and attention to detail, was a stroke of marketing genius, and the popularity of the Dinobots is attested to by the incredible amount of airtime they got on the Transformers cartoon and in "Transformers: The Movie". They were widely and frequently used in the American cartoon from the point of their introduction nearly until the end of the series.
The five Dinobots were Grimlock, Slag, Sludge, Snarl, and Swoop. In the cartoon episode "SOS Dinobots", they were created by the Autobots based on dinosaur fossils found near the Autobot headquarters, which is why they lacked the intelligence of true Cybertronian Transformers. Grimlock, Slag, and Sludge were created first, with Snarl and Swoop added later in "War of the Dinobots".