I made a costume for my son when he was about 4 and obsessed with everything to do with dinosaurs. I had a book called Sew a Dinosaur - 21 playful prehistoric beasts to follow you home, which included a pattern for a kid-sized Triceratops costume.
Somehow or other, I did it.
The body is made without the foam. The tail was a cone of the alligator print sewn over foam. The base of the tail is circular, sewn to the back of the shirt. To keep the tail from dragging, I attached an elastic belt to the SAs on the inside, and it fit snugly around my son's hips. The tail stayed straight out and wiggled as he walked.
It was extremely cute, if I say so myself, and it was worn many times by many kids.