Lihzahrd

A Lihzahrd opens as an upright walker with nothing special about it, closing on you at a jogging pace and swinging on contact. Take it past a little under half health and it drops onto all fours, and that transition is the whole fight. On four legs it moves at more than twice its walking speed, hits harder, takes less damage, and stops flinching from knockback entirely. Anything you were using to hold it at arm's length stops working at exactly the moment it becomes dangerous.

Since knockback is off the table, distance has to come from geometry or from your own movement. The one weakness the crawling form has is a much worse jump, so a platform a few tiles up puts you out of reach while the ordinary walkers can still be pulled in. Do not trust a wall, though. If it transforms while pressed against one it can end up on the other side of it. There is also a stall worth knowing: force one into a gap only two tiles high and it plants itself in place, arms hanging, for several seconds before losing interest and heading off elsewhere.

They spawn on Lihzahrd Brick and Wooden Spike while you stand in front of naturally generated Lihzahrd Brick Wall, specifically wall behind your character's center. Since the walls at the temple entrance are the unsafe generated kind, you can farm Lihzahrds and Flying Snakes from outside the door long before Plantera goes down, and in fact before Hardmode starts at all. That is the well known route to an early Lihzahrd Power Cell, and it is a real fight at that stage rather than a free farm. Remix seed worlds loosen the block requirement so they will spawn on anything in front of that wall.

The Witch Doctor is one of these, which the green skin and tail give away, and his party line about seeing how your kind celebrates confirms he is not human. He is the only friendly member of the species you meet in play.