Hoppin' Jack

Hardmode, and only in two situations. During the Halloween season it spawns at night alongside the seasonal roster. Outside that window it appears in a Graveyard, at any time of year and any hour, so enough tombstones in one place produces them on demand. No summon item is involved either way.

At full health the pause between hops is long enough to stroll away from. Land a single hit and it collapses to almost nothing, and the slow pumpkin keeps pace with you. Chip damage is the worst thing you can do to it: finish it in the first exchange or accept the fast version and give yourself room.

The cycle is two short hops followed by one much taller, roughly double the height. That third jump clears the platform you assumed was safe, so build an arena that beats the tall one rather than the pair before it. It borrows slime movement and nothing else: Royal Gel does not calm it, it leaves no gel behind, and none of the slime achievements count it. The name nods to Spring heeled Jack from British folklore.