Space animal, sometimes contracted as spacie, is a collective slang term that commonly refers to the Star Fox characters (Fox, Falco, and Wolf). The term originated in the competitive Melee community, where players noticed that Fox and Falco not only have an extremely similar moveset, being clones, but also very similar physical attributes, such as high falling speed and gravity, and light weight. These similarities not only grant the two characters a similar playstyle, but also make them susceptible to similar counterplay, such as Marth up throw chain throw in Melee and Pikachu down throw chain throw in Brawl, as well as specialized combos like Jigglypuff's space animal slayer. Subsequent titles differentiated Fox and Falco more, by giving them different moves and slightly altering their attributes; however, their moveset and physical attributes remain very similar across the series, and as such the term "space animal" continues to see wide usage. Super Smash Bros. Brawl introduced Wolf, another Star Fox character with a moveset based on Fox. Although their movesets are only superficially similar and he is an heavyweight in Brawl and a middleweight in Ultimate, as opposed to a lightweight, the overall similarities to Fox and Falco still cause Wolf to be included among space animals.
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, and consequently is occasionally considered by Melee players as part of the space animals despite not being a Star Fox character, not having a moveset based on Fox and not being an anthropomorphic animal, although he is an intergalactic bounty hunter. So at this time the term "Spacie" has come to be synonymous with the term fast faller, which would include Sheik, from "Brawl" and onwards, as well.
Similar to how they all share similar strengths, they also share very similar weaknesses: most of the space animals have poor survivability due to their low weight as well as being very vulnerable to combos due to their high falling speed (Fox having the lowest weight out of the three starting from "Melee" onwards and having the highest gravity in every game, though he didn't have the hightest falling speed until "Smash 4" in the Smash series). They also have very linear recoveries: their horizontal options all travel in a very linear fashion, and Fire Fox and Fire Bird have a very high amount of startup lag, causing them to be vulnerable to opponent's edgeguarding options. Fire Wolf has little startup in comparison, but in Ultimate travels the shortest distance out of the three.
Also, Krystal appears in Ultimate as a Star Fox character and anthropomorphic animal, but due to her actually being a Assist Trophy character and not having a moveset based on Fox, she is not considered as part of the space animals.