Lihzahrd

Lihzahrds are post-Plantera enemies found only inside the Jungle Temple. They initially move as ordinary walking Fighter AI enemies. As soon as their health is reduced to below 55% ( / ), they enter their second, crawling form, which consists of a more lizard-like creature running on four legs. In this form they are much faster, have additional damage, defense, and jump height, and are immune to knockback.

Trivia

 * The Witch Doctor is a Lihzahrd, as can be seen by his green skin and tail. This is especially evident by his party quote, "I wanted to see how your kind celebrate", which would indicate he is not human. This has also been directly confirmed by Leinfors on the Terraria forums.
 * This would make him the only non-hostile Lihzahrd in Terraria.