MC-53166Horses do not convert their owner to UUID
Wolves and ocelots have code that will read an owner name, if one exists, and convert it to an owner UUID. Horses, on the other hand, simply drop their code for owner names and now only work with UUIDs. This means all horses lose their owner when upgrading. Vanilla does not use this information but it does store it and mods have come to rely on it. The /testfor command allows matching an entity with a given NBT tag so command blocks could be relying on it as well.
Created Issue:
Horses do not convert their owner to UUID
Wolves and ocelots have code that will read an owner name, if one exists, and convert it to an owner UUID. Horses, on the other hand, simply drop their code for owner names and now only work with UUIDs. This means all horses lose their owner when upgrading. Vanilla does not use this information but it does store it and mods have come to rely on it. The /testfor command allows matching an entity with a given NBT tag so command blocks could be relying on it as well.
A comment with security level 'global-moderators' was removed.
Mog (Ryan Holtz)
Confirmed.
This still effects the current snapshot, updating a tamed horse from 1.7.4 to 20w29a does not convert the player name, thus leaving the horse without a owner tag.