My eldest son loves making up monsters. Sticks, stones, wire, a rope, a pillow... anything can be repurposed into a monster. 
[Okay I may have been letting him read my collection of Dungeons & Dragons monster manuals LOL]
He made some monsters out of some glow-in-the-dark human skeleton pieces today, recombining them into some pretty creepy creatures! Not to worry, he said these all went extinct millions of years ago in the Jurassic period.
Other times he will create a creature in his imagination, describe it to me and ask me to draw it. Ferinstance, behold the finger-headed butt monster:
When a rubber strap broke off his swim goggles, he told me the broken strap was the goadalist monster. It has a tail, shoulders, wings and a beak. But no body. Can you draw it dad? Sure!
Yes, 
My homemade