Metal Daisy submitted a new mod: Isolation Mod - No More Storyline - Removes the story line, like the beta versions. Read more about this mod...
I hate to drop the bad news but upon opening the mod and looking inside, I see nothing pertaining to monsters or bosses of any kind. So no bosses aren't encountered naturally. Sigh if only someone can make a TRUE sandbox for Starbound with all bosses accessible via summoning or finding, but alas we have no such thing. Looks like I'm going to keep playing Terraria from here on out, while playing Starbound on the side every now and then.