The 1%er first appeared in one of Paul O'Connor's Designer Diaries, and has remained a mystery ever since. What remains more of a mystery is how Xavier got his hands on a larger version of the picture.
Basically, we don't know what it is. First impressions when it appeared were that it was the Slig Queen Skillya, but since then Xavier's larger picture reveiled the word 'Mug' at the bottom, initiating the belief that if was the carnivorous form of the Meetle. This may, for all we know, be the truth. We have seen what Mugs really look like, and it's nothing like this creature, so it could be that this is an early design that was rejected. However, because of its vast difference from the Mug we're more familiar with, some people have begun to suspect it is a species in itself, perhaps a kind of back-street race - muggers, pickpockets, thugs and the like. It certainly look like that with its leather jacket and eye piercings. It's also holding knives in its tail, and its mechanical legs are leaking.
The nickname 1%er comes from a photo of the inside of the OWI studios where the concept art was pinned on a wall under the title of '1%er'. The meaning of this cryptic header is another one of Oddworld's secrets.