Alright, my turn. Got quite a few actually.
Bear: I figured if there are regular pigs, warthogs, bison, dogs, rats, cats, bats and roaches living in underrail (along with all of the crazy mutated stuff), bears aren't that much of a stretch. Basically, they are the epitome of the more "normal" creatures in underrail. Pretty tough, They have decent mechanical and cold resistance and immunity to fear effects. They also have a powerful bite attack, an attack with it's paws which can daze the player and a roar ability which is similar to the yell feat. For feats, they have Conditioning, Bone Breaker, and Pain Suppression.
Screecher: One of the ferry captains mentions rumors of screaming bat monsters while talking about the black sea and that is where the inspiration for this one comes from. The screecher is an oversized, mutated bat with atrophied wings so it can no longer fly. It can do a sonic scream attack that ignores damage resistance and passes thought targets, like attacks from the Sonocaster but also has a chance to deafen the player, which decreases perception and weapon accuracy. It also has a bite attack which inflicts anticoagulation.
Behemoth: I can't be the only one who did the forsaken island quest and now really wants to fight one of these. A big, monstrous lizard that has high health, powerful attacks, and a few feats like Taste for Blood, Expose Weakness, and Cheap Shots.
Mutated Rathound: A rathound that was exposed to potent mutagen gas and horrifically twisted into an abomination. Has less health than an augmented rathound but much more than an alpha. Bleeds acid and is immune to acid and bio damage like other mutants and also has pack hunting and rend like Rathounds. Instead of an acidic spray or acid glob attack, it has a spit attack that releases toxic gas. They also have the Vile Weaponry feat and are not afraid as of fire as regular rathounds.
Mutant Amalgam: A boss that I thought of, kind of like a proto-Tchort if you will. The result of at least a dozen people being exposed to mutagen and fusing together into a huge, crawling, hungry monstrosity. Has barely any movement points but has more action points to compensate. It can spit acid, pick up and throw barrels and rubble, and smack you with it's tendrils. It also has the ability to cast Telekinetic Grounding, Implosion, Psychosomatic Predation, Plasma Beam, Telekinetic punch, Bilocation and Neural Overload. Like Jet Eater, when it's health starts getting low, it will disengage from combat and try to devour one of the several piles of mutant corpses scattered throughout the area. This process takes a couple turns but when finished, it heals 30% of it's health and passively regenerates 50 health per turn for 6 turns. The amalgam bleeds acid, is immune to acid and bio damage and fear effects and it has a lot of health. It is vulnerable to JHP rounds, it's psionic abilities can be knocked out, and the corpse piles can be destroyed so that they can't be eaten.
