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.