Not tested on horses yet but, when I breed two donkeys, then right click the baby with sugar, it goes from stage 1 to the final stage. Teeny weeny baby to adult size.
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kumasasa
MC-73086
Donkey Growth Still Not Working
Resolved
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0
kumasasa
migrated
(Unassigned)
Unconfirmed
Minecraft 1.8.1-pre1
Retrieved
MC-61535 wasn't fixed properly.