a bit of both! technically speaking, the six-limbed variation is a common mutation speculated to be a result of the amphimorpho genome1 having been so highly tampered with through the course of their creation. some amphimorpho are just born with six limbs, and scientists aren't entirely sure why!
of course, being shapeshifters, they can also choose to just have more limbs if so desired~
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the "amphimorpho genome" is a mishmash of various species' genes and a handful of artificially-built sequences, but it is primarily built off of the genome of another alien species, Proteus protomorpho, which basically look like cat-sized amphimorpho. P. protomorpho only has four limbs.
