I must have been about 7. As far as I can remember, it was quite a large, square book with very beautiful darkly coloured illustrations. The story was about a boy called Max (?) who wore a little suit with a tail and a hood with ears on. He goes to a mysterious island/ world/ country and encounters a slightly scary monster who looks rather like the costume he's wearing except much bigger and hairier with big tusk-like teeth and feet with big claws.
Ring any bells with anyone?