Unless the name fits the pattern of "'s Steed" where XYZ != player.getName(), there's no need to restrict people from using the Ocarina to name horses when they can simply obtain a name tag and do it manually.

Security through obscurity/additional steps isn't security at all.

The only reason to ignore "'s Steed"-named horses is to do a small bit to prevent griefing. Indeed, even in that situation someone could probably use a name tag and then an Ocarina to begin griefing, but that would be a server-based dispute which should be resolved by administrators rather than programming.

Actually, this hasn't been fully tested to make sure that non-"'s Steed" horses can be correctly unnamed using the ocarina.

Disregard, tested with the standard testing name of "Boris" on a draft horse mare. She was sufficiently unnamed. Likewise, testing with "Anya's Steed" on a draft horse mare and the ocarina refused to unname it. Simply put, someone else could just screw you over with renaming your horse... but that's not my problem.