Spore Bat

It appears on its own, no event and no summon, but only underground and only where there is a genuine glowing mushroom biome around you. That is the part people get wrong. Most mushroom enemies will spawn off a single patch of mushroom grass, which is why tiny planted farms work for them. This one checks that you are standing inside the biome, so it wants a real spread of mushroom tiles nearby. Spore Skeleton has the same requirement. If your farm produces everything except those two, that is why.

It fights like an ordinary cave bat, so the only reason to seek one out is the drops. The Shroomerang is a strong pre-hardmode boomerang and its drop rate was raised a lot in later versions, which makes digging out a proper cavern biome worth the mining over faking a small patch. The Bat Bat and a Depth Meter turn up here too, both rare enough to treat as a bonus rather than a plan.

The bite it lands shuts off your natural health regeneration for a long stretch and stacks other unpleasantness on top. Mushroom caves are dark and full of ceiling, so the usual sequence is one bat biting you and everything else in the room finishing the job. Kill them on approach rather than after they are already in your face.