Horrid Henry (Dinosaur Transformation)

Horrid Henry is a British cartoon about a bad boy prone to temper tantrums. During some of these he turns into nasty looking animals, though the transformation is entirely imagined. In the show's credits, he's briefly seen transforming into a Tyrannosaurus Rex. 

EDIT: the original link is dead, but all the transformations from the show can be viewed at Shadowlord Inc.