Space Animal is a collective slang term within the smash community that commonly refers to Fox, Falco, and Wolf (commonly also referred to as "Spacies.") The terminology arose out of the study of competitive Melee, wherein the stark similarity between Fox and Falco's physical properties (i.e. fastfalling speed, weight, etc.) can be exploited by other characters in similar ways (see: Space Animal Slayer).
Many of the fundamental similarities between Fox and Falco in Melee are shared by Captain Falcon and he is therefore often lumped into the same category.