Pyton

The "Set animal" aka sha, an ambiguous mammalian creature from ancient Egyptian iconography, never since identified with any extant or extinct species, likely never walked the earth.

Creatures from Max Ernst's 1937 paintings "The Angel of Hearth and Home" and "The Triumph of Surrealism", notably the smaller creature on the left (more toothsome in the latter rendition)

Mural by Leonora Carrington [source], strikingly similar to the recurring birdlike creatures of her collaborator Ernst. Animals and mythological creatures appear frequently in Carrington's work.

Figbash, an ambiguous, faceless, pointy-headed black-silhouette-like creature created by Edward Gorey. Figbash does a lot of dancing and leaping around, posing with wildly pliant arms.