Character: original character
An original character (often abbreviated to OC) is a character that does not exist in the show My Little Pony: Frienship is Magic, even as a background pony. OCs are, as the name implies, original creations of fans. Some have complex backstories; other times someone just wanted to draw a different-colored pony.
If a character appears in the show at all, even briefly, it's not an original character.