Horned Kangaroo

A horned kangaroo is presumably a mutated version of the kangaroo, an Australian marsupial. It is not certain how they ended up in North America - they might have escaped from a zoo.

Quotes

 * "You think these might be Horned Kangaroo bones, but you cannot be sure. They are too weathered to make a final determination."

Appearances
Bones of what the Vault Dweller/Chosen One thinks might have been a Horned Kangaroo appear in Fallout and Fallout 2, most notably just outside the cave in which Vault 13 is located.

Behind the scenes
The horned kangaroo is likely a reference to Mad Max 2, since you can see kangaroo bones in this movie.