There also seem to be simpler fixed-function implants (e.g. NTAG or similar NDEF-storage-only tags), but I don’t think there’s anything non-Java capable of doing something as complex as CTAP2.
For something that potentially needs surgery to remove from your body, I’d go for the most capable secure element you can afford for maximum flexibility and future proofing anyway; usually that’s also Java Card.
Hardware: https://dangerousthings.com/product/flexsecure/
Software: https://github.com/darconeous/u2f-javacard (or https://github.com/BryanJacobs/FIDO2Applet if you're feeling experimentative, which you likely do if you're considering a subdermal Java Card implant)
And unlike a Yubikey, this one's even field-updateable :) (At least the applications; not so much the Java Card and OS)