Amalgamates

The Amalgamates are a group of monsters brought back from the dead by Alphys by infusing them with Determination. Unfortunately for them, monster bodies were not meant to handle the power, and as a result, the magic that composed them lost cohesion and fused together into strange, alien, and often horrifying creatures after Alphys prematurely promised their families she would return them. Ashamed of her failure, their creator confined them to the True Lab, where many have apparently regressed to a semi-beastial state, attacking Frisk in the belief they have food while exploring the lab. Despite this, they aren't heartless like Alphys' other accidental creation, due to still possessing souls. If Frisk lies on the uncovered bed for a while, one will even tuck them in kindly.