Experienced neopet are cards that you keep in your regular deck. They are sort of like upgrades to you basic neopet of the same species. They have better stats than basic neopets, and most experienced neopets have an effect. To play an experienced neopet, you need to have a certain type of basic pet in an arena. For example, to play a Kacheek Shepherd, you need to have a basic Kacheek on the field and a light neopet. The basic neopet then becomes that experienced neopet. So a Blue Kacheek would then turn into a Kacheek Shepherd when you played the experienced neopet.
Heroes are cards that are like experienced neopets except you don’t need a basic neopet to play them. Very similar to experienced neopets, except you need to tap a neopet to play them. Heroes usually have good effects and great stats. Heroes are discarded after every contest.
Villains are kind of like a Hero and Experienced Neopets combined. You don’t need to tap a neopet to play them, but you do need a certain type to play them. They have good stats like Heroes, but unlike Heroes, Villains usually have bad effects. And also, villains are discarded after every contest.