It's NOT Denver the Dinosaur I know that much.
All I remember is he could open portals to other dimensions/times(?) and kept showing up at the worst times and the kid was always keeping him hidden from his parents. I think he had blonde hair and I remember the animation looked fairly good for its time, modern/future-y sort of art style.
The dinosaur (or monster?) was green, fairly big, could obviously talk and tended to be a pain in the ass. I remember the kids house looked pretty cool too, super modern looking.