The reason there are so many different variations in the different monster species is because of how monster genetics work. When two different monster types get married and have a baby, the newborn monster is the same species as one of its parents, inheriting the body shape and some other typical traits of that species such as the number of eyes. The rest of the young monster's traits (such as size, colour, horn shape, wings, fur, number of arms, etc.) are determined by its genes.
- Terri and Terry probably inherited their body shape from their mother, making them the same species as she is. Other traits they got from her were one eye and Terry's horns. They inherited their yellow colour, tentacles, and Terri's horn from their dad. Having two heads and four arms was by chance.
- Claire's mother is a Flapwinger like the PNKs, therefore Claire inherited the Flapwinger body shape, three eyes, and natural fur skirt from her. Her fin-like hair, three toed feet and tail are traits from her dad, who is an aquatic monster.
- Sulley is most likely the same kind of monster as his dad. He inherited the body shape and horns. He got his fur colour from his mom.
If the two married monsters are the same kind, their baby will be a pureblood individual of that species. Mike and Squishy are examples of this.
In the comments below, put a monster's name, what their parents might be like, and what traits they inherited. Remember, the monster's parent who has the same body shape as their offspring is also the same species.
Please refer to this chart for a better description of what I'm talking about.