Most monstrosities I think of an animal as a base with magic added on, so the chimera is three melded animals with some magic powers. A displacer beast is a six legged panther with shoulder tentacles and a magic power.

A roper looks like it is not at base an animal, other than having an eye and a mouth. Aberrations can have weird anatomy and body consistency. Enough mutational transformation will turn things into aberrations.

My conceptions of the categories were formed a lot by the 3e type definitions and a lot of the original categorization done in 3e.

"An aberration has a bizarre anatomy, strange abilities, an alien mindset, or any combination of the three."

"Magical beasts are similar to animals but can have Intelligence scores higher than 2.
Magical beasts usually have supernatural or extraordinary abilities, but sometimes are merely bizarre in appearance or habits."

A roper fits more into an aberration definition to me than a magical beast one.

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