I haven't played too much yet, but just as I was getting used to a specific boss and knowing how to focus my builds to defeat it, I was shocked to challenge a different boss.
I've now challenged three different bosses overall, but I can't seem to find any indication of who I'll be fighting until the first hero makes his/her first attempt.
Is there, in fact, any way to know what boss will appear before challenging it, or is it intended to be unknown until it's encountered?
After doing some more digging, it appears that there is no way to tell what boss you will encounter until you enter the portal to fight it and the boss is randomly picked for each game.
From this review:
Everything about the game is randomly generated — the dungeons, which monsters show up, which boss you encounter, and what's for sale in the shop — which helps to keep things interesting.
Additionally, from this review:
Later on, the hero must fight a boss, randomly a three-headed hydra, a tentacle beast, or a rocky, underground demon.