A person consumed by intense rancor or envy is referred to as ardilla (ar-DEE-ja, lit. squirrel). It is a derivation from ardido, ardida, which is the past participle of burn. It is an allusion to the fire-like sensation inside one's guts that such feelings produce.
Calling someone ardido is a candid way to express disapproval for hosting such mean sentiments. It is typically used in the phrase ¡No seas ardido! (don't be burned!) or ¡Ese güey es bien ardilla! (That dude is very rancorous!).