Which standard organization (e.g. NIST or ANSI) standardizes signature Ed25519?
In December 2015 there has been a followup on FIPS 186-4 where multiple organizations could respond to a few questions about that document from NIST. Many of them indicated that they would like Ed25519 to be supported (and Brainpool curves and Goldilocks-curve as well). There has been no followup 186-4 yet, so that means it is not in there (yet).
ANSI X9.62 and X9.63 were heavily influenced by Certicom (that use the same set of P-256 etc. curves). It's darn unlikely that they contain Ed2219 by now. There is no sign of it anyway, and the latest official X9.62 predates the official release of the curve by D.J. Bernstein.
In other words, it hasn't been standardized by these US based organizations yet.
None yet, but the draft FIPS 186-5 and draft NIST SP 800-186, currently awaiting public comment as of 2019-10-31 and until 2020-01-20, include the Ed25519 signature scheme, along with essentially everything else in RFC 7748 and RFC 8032—except for the X25519/X448 DH functions, which will likely be included in a future update to NIST SP 800-56A.