I agree, it certainly looks like "erupting" mobs that are created and triggered on an action (e.g. moving to a certain spot) are as normal, but no pre-placed mobs have a chance to be champions right now. The only mobs I've seen as champions on low level play are the bats and erupting-from-the-floor skeletons in The Catacombs, which both meet this pattern, as are undead spawning from broken sarcophagi.
(I much prefer play with Champions, so this is an inferior experience for me)