Eternal Hive vomits 2 flies at the player instead of 1, and bursts into flies, Fat Flies and Spiders
on death.
[[1]]
There's also one Miniboss version of Eternal Hive. It appears larger than Eternal Hive, has 2 Eternal Flies surrounding itself, and vomits Eternal Flies while running away from Isaac. It explodes into several Eternal Flies and Spiders on death, and will always drop an Eternal Heart.