Elf in personal names is an enigma, given the contemporary concept of elves (they turn the cream, spoil the milk, make people and animals sick in body and mind, wither the crops etc.). The other "monsters" used in dithematic names (thurse {giant or "long man"} and worm {dragon}) are tribal totems, but elves were immaterial, malicious spirits. It's possible it didn't originally refer to the evil spirits at all — the word is identical to the name of the
German river Elbe and literally means "white", cognate with Albion, Alps etc. — as spirits elves, like ghosts, are white, and the Elbe at its source is the "white" river. Of course other themes drop out of use once they acquire negative meanings in prosaic language, so it's a mystery why "elf" persisted onomastically in an era when they were viewed so negatively.