The Iguana is a neutral creature that is usually has 1-3 friends around. If you attack one, ALL of them will swarm you, like Zombie Pigmen. This could end up with critical damage. They can swarm in a manner rivaled by Silverfish.
When fighting, be prepared to be attacked by a couple of these creatures. Then will run around your feet, and jump at your face, causing damage. Their hit box is small, found around their neck. A way to stay safe is to build a 1x2 pillar, and attack from above. This will keep you from getting swarmed.
Edit
