Near tunnel there is a super mutan camp and a train car of whatever it's called. Inside should be cages made out of shopping cart. Inside one of them is teddy bear. Pick him up and it will spawn a behemot (no, this one isn't revelant to the one in evergreen mill)

Thanks for the heads up, but this is isn't my first playthrough. ~8 years ago I did a 100% good guy run trying to clear every possible location (and DLCS) which took me 70+ hours.

Now I'm trying to do the opposite with a bad girl character trying to focus to main quest and avoid side quests. ( I'm not sure if my character has enough motivation to do DLCs.)

But I wasn't able to find information how these unique enemies would behave, if you meet them but then run away.