Fiestacat is a cat who's job it is to party, or be involved in "fiestas", he wears a party hat, 3d glasses, and smokes from a pipe (the contents of which are a matter of debate.) The image of fiestacat can be printed on posters which are then posted on any door or window as a sign to others that a fiesta is about to occur at that establishment and partying hard will commence.

Got the covid-19 vaccine

Cat with 3D-glasses, silly hat and smirk grin. Widely considered the epitome of hilarity.

A cat in a jester hat smoking a pipe and wearing 3D glasses. Also referred to as "that cat".
Though rarely found in big internet forums, fiesta cat is commonly seen in UTSNet, and is often accompanied by phrases such as fyad, zany, 5ive, and top post.
Whence the cat came from is unknown.