Pony.fm is a streaming service for and by Bronies, and they have support for lyrics on their track pages. some examples:

• https://pony.fm/tracks/45647-brush-addict-ft-nyancat380
• https://pony.fm/tracks/46542-friendship-in-the-aire

I haven't confirmed yet, but I believe these are added by the artists themselves, as there's no way for me as a regular user to edit them, so it seems like a good source to me.

URLs appear to be formatted as "pony.fm/tracks/[ID#]-[track-name]", and I believe any other URLs should be rejected from this relationship (there's also album, artist, and playlist pages, which aren't under /tracks/)