Since when did frenzied boar boss packs spawn?

[Baajikiil]

And can they be found exclusively in Hell mode?

They can be found in NM as well. They are fairly uncommon because of all the guest monsters in act 5. If one of the random monsters selected for WSK lvl2 is suicide minions, you will often find several boss/champ packs of them on the lvl. I've seen 2 suicide boss packs spawn together, right at the wp. Quite amusing to see them all chain explode on my merc as he charged them. I think I have a few screen shots of different interesting packs of them. Tough to take the screenshots though, as I'm fleeing for my life(HC).

Too bad they don't drop anything.