Cosplay rules are pretty simple:
One cosplay per species. Period.
Once more stock lines come into play, this will not change at all.
First come, first serve.
You cannot "hold" cosplays.

These count as one-per-species cosplays:
-Pokemon
-Digimon
-Book, TV, movie, manga, game, etc named characters
-"Celebrity" entertainment personas, like wrestlers, magicians, etc

These do NOT count as cosplays, and there can be infinite amounts of them:
-Real-world animal breeds or species
-Fictional species such as Na'vi, WoW animal breeds, etc

As far as fictional species (with the exception of Pokemon/Digimon) go, if it has a name, it's a cosplay. If it doesn't, it's not. So, while a named character from Avatar would be a taken cosplay, a generic Na'vi colored pet wouldn't. One of the named Dragon Aspects from WoW would be a taken cosplay, but a generic bronze dragon colored pet wouldn't be a taken cosplay. etc.

These are the taken cosplays.
Bears
Cubone
Slowpoke
Bulbasaur
Lickitung


Sabers
Meowth
Pikachu
Dragonite
Caterpie


Wolves
Gastly
Ninetails
Arcanine
Vulpix


Horses
Ponyta
Pidgeot


Mammoths
Tauros