Giant Tortoise

Jungle, surface and underground both, hardmode only. The ice biome has a version that behaves identically under a different name, so learning one teaches you the other. Nothing summons them. They wander the jungle floor at a plod and look completely harmless right up until they are not.

It tucks into its shell, then hurls itself at you with much higher defense and enough contact damage to delete an underprepared character. The direction locks in at the moment it retracts, based on where you were standing then, so a sidestep beats it. It is not blind afterward though. It knows where you actually are and will brake hard and drop straight down once it is above you.

The move takes a beat to wind up and a couple of seconds to travel, and any damage you land during it cancels the whole thing.

The cooldown between spins is the one mechanic worth understanding. Up close it barely uses the attack at all. Back away with a clean line between the two of you and the gap between spins collapses to a fraction of what it was. Get properly far off and it turns into a machine gun. Running away is the worst possible response.

Damage also resets the timer. Anything with a high rate of fire, a yoyo, fast thrown daggers, or a minion that keeps chipping at it locks the attack out completely. That is why a Megashark trivializes it and a slow heavy swing does not, despite the heavy swing looking better on paper.

Two things about the ground matter. Water is the trap: a spin that ends in a pool immediately starts another one, so fighting near a jungle pond means taking charge after charge with no gap between them. And they cannot fall through platforms. A few long rows of platforms above the floor turn the fight into a slow walk toward something that physically cannot reach you.

You need Turtle Shells for the turtle set, and the surface jungle is the better place to farm despite feeling more exposed, simply because there is more open ground for them to spawn on. Underground works if you hollow out a wide chamber first. A Pirate Staff does the job in either spot, since constant minion hits keep the spin suppressed while you stand still and let it happen. Bring defense or a dodge effect regardless, because one spin connecting on expert or master is most of a health bar.