In Toriko, what animal's skin does Atom show Terry, Kiss and Quinn?
Unknown creature's skin: ATOM possesses a mysterious piece of skin that appears to have belonged to a creature of tremendous power and vicious bloodthirsty ferocity. Just showing this little piece of skin could invoke paralyzing fear in the hearts of Terry, Kiss, and Quinn.


Taken from Toriko Wikia.

So, it seems never to be resolved from what kind of creature this skin was.