RGMII - Origin of RGMII and does a standard exist?
RGMII was born the same way the original Ethernet was.  Some interested parties (businesses) brought together and prepared a specification (a type of technical standard) for the MAC-to-PHY interface they were in need of.

Why did not they send it to a (inter)national standardization body like IEEE, ISO, IEC, ITU, other "official"?... My guess is because the "official" standardization process is (very) costly (and also not so fast) and the interested businesses did not see any extra profit resulting from the "official" standardization.

Despite of this, RGMII is very widespread today. Maybe it is not a de-jure ("official") standard but thers is no doubt that it is a de-facto ("useful") one.