Go on some random single player world.
Spawn a horse.
Use a name tag to name this horse - Grumm
Remember that Grumm has a capital G and you must spell it exactly as shown
Once the horse is named Grumm get ready to laugh like crazy!

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This works on ALL mobs! Not just horses.
So far Ive seen no game-breaking effects of this glitch/bug/secret
Im not gunna tell you what happens so go try it yourself.
To undo the effects just rename the mob something else
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hope you try this out! im not sure if this works on servers yet. :D