The Amalgamates

The amalgamates are a group of friendly monsters who, despite being normally docile, look terrifying. They are multiple monsters who melted together after being injected with Determination by Dr. Alphys, a substance which is believed to allow souls to persist after death. All of the amalgamates are amorphous and can melt completely and then return to their normal forms. The amalgamates are:
 * Endogeny
 * Lemonbread
 * Reaperbird
 * Snowdrake's Mother
 * Memoryhead(s)

Appearance
Endogeny appears to be a giant, slimy, white dog with a single oozing hole where its face should be. It has six legs with clawed feet. Lemonbread looks like a dripping mermaid-like creature with muscular arms and a reptilian head. Reaperbird is a bird with a horned head and one eye on located one side of its face. It has a long neck, crooked wings with holes in them, and enlongated legs. Snowdrake's Mother is a small, birdlike creature with smiling faces for eyes, and a snowflake-like shape on its face with an eye on top. Half of her face is dripping off. The memoryheads appear to be oozing white masses of many faces stuck together.

Story
The amalgamates are only seen in the past during Act II, where they are captured along with Alphys, Mettaton, and several of the Avengers. They are killed by villains in the nomu gauntlet after they save the heroes, and although no Amalgamates join the Final Battle in Act III, they were never previously on the team and it was never specified if they even disappeared in the first place.