Crocodiles are scary. I remember reading a short story as a young man where a heart surgeon discovered crocs had over centuries, developed a chronic cardiac condition that left them comparatively sluggish and 'weak. He kept operating on them, fixing the problems until a few survived the surgery. As they recovered and grew bigger and stronger, they developed protuberances on their shoulders and a hard pointy plate at the end of their tail. You can see what's coming - they eventually turned into dragons.. They story was told in retrospective, from the safety of underground caves where what was left of humanity lived now..

Always crossed my mind when I see one :)