The Duck of Many Things
This delicious bird, much sought-after by gourmets, is able to polymorph at will into other creatures and ordinary objects in an attempt to evade capture. Examples that have been recorded include a small whale, a giant frog, a clump of reeds, a floating log, a swarm of mosquitoes, and a rusty pen-knife.
The beak of one of these, if powdered and mixed into a paste with tincture of verdigris, can be used as an alternative material component for casting the Polymorph spell.