The origin of the name is obscure too, though the UK Dictionary of National Biography claims that it is "probably of Frankish origin" and an individual of that Rahere was mentioned in court records in Brittany. Ancestry.com., in relation to the name 'Rayer', suggests that it comes from "the Norman personal name Raher probably from an ancient Germanic name composed of rād ‘counsel’ + hari ‘army’." The name 'Rayner' has that meaning, so perhaps it was an obscure variant of 'Rayner'.

It's very rare, he appears to be the only one historically known.