Space animal, sometimes contracted as spacie, is a collective slang term that commonly refers to the Star Fox characters (Fox, Falco, and Wolf). The terminology arose out of the study of competitive Super Smash Bros. Melee, where the stark similarity between Fox and Falco's physical properties (most importantly high falling speed, high gravity, and light weight) can be exploited by opponents in similar ways, such as in chain throws like Marth's up throw chain throw in Melee and Pikachu's zero-death down throw chain throw in Brawl. This also applies to specialized combos well, as in the case of Smash 64 and Melee, Jigglypuff's space animal slayer. However, having light weight isn't a consistent property of a space animal, as in the case of Fox in Smash 64, who was a middle weight, but possessed high falling speed and gravity. Wolf was added when he was introduced in Super Smash Bros. Brawl, having high falling speed, high gravity while at the same time being a heavy weight.
In addition to the three Star Fox characters, Captain Falcon also has high gravity and high falling speed. As a result, Captain Falcon is vulnerable to many of the combos and chain throws that afflict the space animals. He is sometimes considered part of the space animals despite not being a Star Fox character or animal, although he is an intergalactic bounty hunter.