I was in elementary school in the 70s and I remember reading what I think was an early chapter book series with a boy and his purple dragon/dinosaur/monster. The illustrations in my memory were very minimalist and had a Quentin Blake feel to them but I’ve researched Quentin Blake’s work and don’t see it listed. The dragon/dinosaur/monster was very tall, definitely purple, and walked on two legs. I have a vivid memory of the boy riding on its shoulders. I don’t remember anything about the plot except they seemed somewhat surreal (maybe?) so I don’t know if the creature was real or imaginary. It’s possible the series was school literature only (not mass market published).