An ugly monster. Lisa Simpson once said it.
"some of them are gorgeous, some of them are plain, a few are ugmos"
Or, from The Simpsons:
Lisa: I'm an ugmo.
Homer: Now, that's not true. You're cute as a bug's ear.
Lisa: Fathers have to say that little stuff.
Homer: Dad, am I cute as a bug's ear?
Grandpa: No. You're homely as a mule's butt.
Homer: There. See?