Jungle Key

The key does not come from one specific enemy. In Hardmode, anything that drops coins and dies while you are standing in the Jungle can roll for it. What decides the key type is the biome you are in, not the creature you killed, so a surface enemy that chases you into the Jungle counts the same as a jungle native. Meteor Heads are the single exclusion.

Because the roll is tied to dropping coins, several whole categories are dead ends. Critters give nothing. Neither does anything a statue produced, nor most of the minions a boss summons, nor the raiders that arrive with the Old One's Army, nor anything a Lunar Pillar puts on the field. That rules out a few of the farms people normally reach for when they want volume.

Keys start dropping from the first minute of Hardmode, long before there is anything to do with one. Until Plantera has died in that world, the key sits in your bag with a note about a curse and the Jungle Chest stays shut. The lock is a property of the world rather than the key, so a key found in a fresh world will open the chest in another world where Plantera is already down.

The odds are bad enough that volume is the only real plan. Put your arena or your spawn point inside the biome so everything you fight, including events and boss attempts, rolls for the key while you are busy with something else. Snow is by far the most comfortable biome to live in for this. Overlapping biomes are worth knowing about too, since a kill in one can roll for either key and very rarely hands over both. Artificial biomes count as well, which is the only route to a Crimson Key in a Corruption world.