When Tommy Oliver and Anton Mercer had their lab invaded and destroyed by Mesogog, some of their experiments were scattered across the outskirts of Reefside. In the episode Wave Goodbye, Tommy found three eggs (color coded to match the three rangers at the time) that he recognized as having unborn Raptor Riders in them. He took the eggs back to his laboratory in his house's basement and began to tame them.
Later on, when the Rangers needed a way to chase the monster, Tommy sent the Raptor Riders, which allowed them to catch up with the monster before having them resort to their zords.
Despite being introduced early in the season, they were only rarely used, due to them being made obsolete by the Raptor Cycles just two episodes after their first appearance. They weren't seen again until Burning at Both Ends.
The Raptor Riders are color-coded to match their ranger. They are mechanical in appearance, but resemble velociraptors (hence their name), and appear somehow as a sentient creature.
Raptor Riders are able to easily scale walls, jump buildings, and run at speeds that even the rangers in their morphed state could not reach; however, the speed at which they could run still paled in comparison to those of the ranger Raptor Cycles.