I think it was called 'Thing' or something?
It was a book I bought with my pocket money at a sale my primary school were having and it was about a girl who have a tiny stegosaurus which some spoiled lass stole because it was the only thing that someone else had that she never had.
I still remember the drawings of this wee stegosaurus hiding in a trash can or behind a tree. It was super cute, I must find out what it is to buy for my son.
Just looked it up as I was about to post this, it's called Thingnapped by Robin Klein.
Can't find it in the UK but when I have the funds to from overseas, I totally will.