Young Tiger Swallowtail butterflies aren't very pretty. They are one of the large caterpillar species that outgrow camouflage, as a defense, and depend on repulsiveness to discourage birds from eating them. They have different looks as they mature; young caterpillars look a bit like bird droppings, but older ones try to look like little snakes. Body segments behind the real head are thicker than the rest of the body and are marked like a little snake's head. Retractable fleshy "horns," called an osmeterium, can be waved about like a snake's tongue.
If you can stand a bit of a gross-out, here's a YouTube video of a mature Tiger Swallowtail caterpillar looking for a safe place to pupate:
And here's a Tiger Swallowtail caterpillar seriously trying to scare away a pesky child: