There are underlings, the most common types. Then we have the specialists that have a lot more HP, like the railgunners, the necromancers, the screechers etc. and then we have the champions that are the end bosses of expeditions and can appear even with a white nametag on the map.

Thanks @dukefx

So what are the much rarer mobs that have their own icon, they have much higher HP pool and multiple skills?

I think those are the untainted you are talking about

Hmmm, they appear randomly very rarely during expeds, even had one appear during Act 3 when getting through the walls.