I am creating a drow monk character who, to oversimplify things, was altered by an ex-member of the Simic Combine in order to put them in a sort of Witness Protection. For this question, the only two details that are important are that she's a drow who still identifies as an elf, and she was enhanced with animal parts, because the more I thought about it, the more I realize this race is a D&D version of a classic philosophical question:
"How many enhancements do you need before you stop being human?"
In the description of the Simic Hybrid race (GGtR, p. 20), it states:
The Simic Combine uses magic to fuse different life forms together. In
recent years, the Simic Combine has extended this research to humanoid
subjects, magically transferring the traits of various animals into
humans, elves, and vedalken. The goal of the Guardian Project is to
build a Simic army of soldiers perfectly adapted to a variety of
combat situations. These hyper-evolved specimens are called Simic
hybrids, though they sometimes refer to themselves as guardians.
Also:
A hybrid's biological enhancements can change its appearance
drastically, though most hybrids retain their basic physical form. All
are augmented with characteristics of animals, mostly aquatic,
reptilian, or amphibian creatures.
And:
Animal Enhancement. Your body has been altered to incorporate certain animal characteristics. You choose one animal enhancement now
and a second enhancement at 5th level.
So putting all this together, it seems like the being would still be fundamentally an elf, human or vedalkin, simply with additional parts they have control over (Kinda like Spiderman's Doctor Octopus without the mind control)
If this is the case, then would my drow still be able to reach her racial feats, or has D&D determined that one enhancement process has no longer qualified her to identify as an elf?
Similarly, the Elf race has it's players starting with one known cantrip under the idea that you've been so exposed to magic, you learned to do one party trick, but do it really well. Since you become a hybrid as an adult, does this mean the Simic Combine stole your ability?
Best Answer
No, Simic Hybrid is its own separate race.
A player character in D&D 5E gets one choice of race, such as human or dwarf or elf. Mechanically speaking, the Simic Hybrid is a race, with its own speed, senses, ability score increases, and other racial traits. It is mutually exclusive from other character races.
If a creature somehow becomes a Simic Hybrid (such as through reincarnate or the DM making stuff up), then their race is effectively changed. They would lose all racial traits of their original race, and instead gain the racial traits of Simic Hybrid.
So a character could not be both a drow (elf) and a Simic Hybrid. For narrative reasons, you could describe your Simic Hybrid character as being part drow, but mechanically they would only have the racial features as described in the Simic Hybrid race, and not the elf features.