Basilisk

It only exists after the world turns Hardmode, and only in the underground desert. Nothing summons it and no event brings it out. It takes over the job the Antlion Charger had before the shift, doing the same thing with far more behind it, so a stretch of desert you cleared comfortably last week will not go the same way.

It runs at you. That is all of it, and it hits hard enough that one missed dodge in early Hardmode takes most of a health bar. Knockback barely registers, so you cannot stop a run with damage the way you can with lighter enemies. You have to be somewhere else when it arrives.

Height is the simplest answer. It needs floor to run along, so anything that lifts you off the ground of a tunnel turns the charge into a non event. On foot, walk backwards while you fire instead of holding position, because the fast guns available at that point are accurate in the middle distance and sloppy past it, and standing still just donates the gap to it. A minion chewing on it while you retreat takes most of the pressure off.

The Ancient Horn is why anybody hunts these on purpose, since it turns the thing that has been running you down into a mount you ride. It is rare, so the hunt is a grind, and the underground desert in early Hardmode is a rough place to spend that time. Coming back a couple of armor tiers later is easier than forcing it. Sturdy Fossils come off every kill regardless.

It has almost nothing to do with the basilisk of European legend, which was a serpent that killed with a look and poisoned the air around it. The sprite is much closer to a thorny devil, a real lizard from the Australian desert. Its internal name in the game files is DesertBeast, which is more honest.