Press and hold to drag a body template on a message and then start creating! Add eyes, props, and expressions to create your own creature right on top of your friend's messages. Watch the App Preview to see an example.
Disclaimer: These images use Apple's sticker technology built into the iMessage platform. Creatures are composed in the conversation window by dragging individual stickers on top of each other, or on top of already sent messages, not in the message composition window.