The Predacons were a gestalt team of Decepticons added to the Transformers toy line in 1987, the same year "Transformers: The Movie" came out. While they didn't appear in that movie (it took two years to produce the film, and when the story was written the Predacons either didn't exist or weren't certain to be introduced as toys), they did feature in the Transformers cartoon immediately following the movie, starting with "Five Faces of Darkness".
The Predacons -- Razorclaw, Rampage, Tantrum, Divebomb, and Headstrong -- were all African animals (but not all predators, despite the name), and were unique among the post-Constructicon gestalts because all five were the same size and price. Their higher-than-average cost often kept kids from being able to collect the entire team on their own, and this is probably a big part of why they sell so well on eBay even today.
When the Transformers were recreated as Beast Wars,
Autobots and Decepticons became Maximals and Predacons, and now "Predacons" was a unifying term for all the bad guys. Later in Beast Machines, the Predacon army was fortified with Vehicons, evil Transformers that changed from robots into vehicles instead of beasts.
When Transformers: Robots in Disguise was introduced to American shores, the beast-transforming Predacons were back again, but this time the vehicle-mode villains introduced later were called Decepticons. The good guys were Autobots once more and were all land-based vehicles. Whether they're the same Predacons we saw in Beast Wars/Beast Machines has yet to be conclusively decided.