These are the terms I used to distinguish different types of mutations in the Underground. In this setting, they are recently evolved from earthlings and still appear very human.

Psions and mosaics can pass as human while therians and chimeras typically cannot.

Psions physical bodies typically grow to adapt to their abilities, i.e. someone with super strength would have denser bones. the line is drawn at their physicality staying within the realm of "human"

Therians and Mosaics are a relatively "young" types of shapeshifters, in that their shapeshifting is generally inefficient, uncomfortable and even painful compared to other types of shapeshifters in the galaxy (Ash's guys). Once a Therian or Mosaic has shifted, they're committed to that form for as little as a few hours up to a few weeks before shifting again, to let the body recover before the next shift. The process of shapeshifting is slow (as in, it's not instantaneous or could take up to several minutes).

Therians are limited to their two forms (human and non-human), their shapeshifting less complex than mosaics and is relatively more fluid/stable as a result. There is no partial shifting into hybrid forms, once the process has started it doesn't stop until it's finished. Most Therians have features of their non-human form when appearing human (depending on the individual), usually in the form of surface level changes, soft tissue and extra limbs/appendages - these features generally don't given them any advantages and they are still considered "human." Unlike mosaics, Therians cannot gain/lose mass when shifting. A Therian's alternate form is determined mostly by genetics with minor deviations that don't settle until puberty. It's generally believed that the more "practice" a Therian has with shapeshifting, the more control they have over their changes.

Mosaic shapeshifting is mimicry of other species biology and abilities, usually through physical contact. They can only take on one form at a time (all or nothing, no cherry picking what they want), but can copy anything and gain/loose mass as needed. Mosaic shifting is less stable/inefficient, takes more time and energy to fully shift (the idea being that they have to "create a new blueprint" instead of switching between two existing like Therians), exhausting the shifter in the process. Most have learned to plan their shifts as uncontrolled/unplanned shifting can be dangerous to the mosaic. Once shifted, they tend to remain in that form for at least a few days, if not a few weeks before attempting to shift again - they could also stay in one form indefinitely. Will always look like themselves regardless of the species they are mimicking, as in, they can't change their "core appearance."

Chimeras pretty straight forward, non-human features, no shapeshifting, no "super powers" beyond their physicality. Appearances range from looking relatively human to not human at all.

Any number of things done to the body to increase quality of life/survivability in space and on different planets. Cybernetics and simple/temporary biological modifications (i.e. vaccinations) are relatively common with high (if temporary or less potent overall) success rates, there's a standard procedure so they can be mass produced safely (and to accommodate different biologies). This opens up space travel for the average person.