Im guessing you have actually beaten the bosses on this world (have to clarify, some make two worlds where they beat the bosses on one and hang the heads on the other for the powers without bumping up the raid progression by actually beating bosses)?

If so, both the Bats and oozers/blobs raid events can start happening any time after you beat Bonemass, and last I checked Bats can happen in any biome while the oozers one happens in meadows, black forest, swamp and plains. Even if they aren't normally from that biome, the raid events can bring them there. (Edit: the bats event requires not just bonemass, ut for you to have killed a bat as well...a bit like the troll raid CAN happen after Elder, but only if you've also killed a troll).