EDIT EDIT: sorry, i meant to post these about 4 hours ago, but i had to leave and didnt get back until now....
EDIT: ok, Ive found some... I looked for pets that are so ugly that they are cute. I looked for purple and black as main colors, but also with primary yellow. I also looked for things with bushy ears or a long scaly looking tail.... basically cute monsters that match. this is what all I found...
Mutant Petpets
(Seeing as the Usul is mutant, a mutant petpet could be the best companion for an ugly neopet)
Mutant Angelpuss
If you are looking for one of the basic petpets with a twist to match your mutant usul, this is a great match, for it may be sorta green, but it has the same eye as the usul and has those purple wings which match the usul.... a decent match.
Mutant Peo
If you are looking for a more "cool" mutant petpet, this one is great because it is purple, with the yellow eyes. Its a very original petpet, so this is definetely a "cool" match.
Mutant Peadackle
If you want a different or original petpet that not many people have, this one is great. The peadackle has always been a funny petpet, and its mutant, purple, and has yellow eyes. A funny match that should make people smile whenever they see it.
Monsters/Little Evil Guys
There happen to be a lot of little devils that are great petpets for a mutant usul. Little monster with teeth, claws, and the brain of a two year old. You can just picture any mutant petpet having fun with these little petpets.
Drackonack
Another classic petpet that is also one of the most monster-ish petpets on neopets. Its an evil little demon, but it is purple with black as a secondary color, so it matches the usul well. It also has the little black claws. A scary, yet practicle combination.
Drackobunny
Possibly the only thing scarier then a drackonack, because of its irony. Not many people have drackobunnys because they arent practicle for many Neopets. However, this is a monster that is attempting to be cute, and it may not be "mutant", but it sure looks like it, so this is great for an original petpet.
Snorklin
Looking for a monster thats... a little nicer looking? A snorklin is nice and evil looking at the same time. Snorklins are always cool, and is purple with yellow eyes, so it matches
Gnar
Evil Crazed Maniac petpet thats purple with yellow matching eyes... what else can I say?
One eyed petpets
Your neopet has 3 eyes... a petpet should be a lesser form of your neopet.... so what better then a one eyed petpet?
Meowclops
The meowclops is a decent match because its a basic cat, and its greyish black, which is the usuls secondary color. It also has the one, big, yellow eye. So, while the colors arent a perfect match, the overall match is cute!
Cyodrake
The Cyodrake is a very cool petpet. It has cool written all over it. And, its purple, and has one eye.... so, its overall a cool petpet for a mutant usul.
Black Cyodrake
Black also matches the usuls secondary color, so like the plain cyodrake, this is a cool match.
Cuter petpetss
So theres always the chance you dont want a monster... how about just a cute petpet that matches... thats NOT a mutant.....
Halloween Greeble
The halloween greeble has always been a funny petpet that makes people smile... i mean come on, its a frog dressed like a spider! Its blackish/grey, the usuls secondary color, and has big yellow eyes. Also, mutants probably hang out with the halloween folk, so this could always be a decent match.
Piraket
This may not necessarily be
cute, but the colors match, and its got a cool attitude.... its purple, with a yellow beak. Pirakets are always mysterisous petpets that catch peoples attention because they are different, so this could be a neat petpet as well.
Mazzew
The Mazzew has always been a classic cool petpet *since it was revamped from that hog looking thing* and now it would be a cute match since its purple, and.. well, that may be all it has, but it still looks darn cute with a mutant usul.
hope that helped!