My original concept would actually have been better as an Aura, probably along the lines of
 
Honorary Yakhood (W)  
Enchantment - Aura  
Enchant Creature  
Enchanted creature is an Ox in addition to its other types and cannot attack if its power is 3 or less.
 
The problem with this was 1) I wanted it to be an equipment; 2) my wife took one look at my card and mentioned the Horny Helmet from Munchkin, and so I was drawn to a parallel; 3) my original Pinkie Pie was 5/1, so it the “heaviness” needed to be based on toughness.
 
Then when I hit 2, it was a problem of finding a way to give natural yaks the extra bonus, while giving non-yaks a smaller bonus and making them yaks without also getting the yak issue.
 
I almost didn’t post it because of whay @The Hittite says, it is too complex. In the end, though, I was so moved to make this into a card when I saw the episode that I didn’t want to back down. I really like the @FaustXXXVII idea, though!

@FaustXXXVII  
Makes sense. Only prince yak can give honorary yak horns!

I think it would be nice to have something more like:
 
“When NAME enters the battlefield you become the monarch. Equipped creature is an Ox in addition to its other types and has +1/+1”

Yeah, dropping the yakification would probably be a wise move. I suppose it’d be something like this…
 
Equipped creature gets +1/+1.
As long as equipped creature is an Ox, it gets an additional +2/+2.
Equipped creature can’t attack unless its toughness is 4 or greater.
Equip 3

Not that good, honestly; practically dead outside of dedicated yak tribal. Maybe 2 to cast, 3 to equip, +2/+2 with a bonus +1/+1 for Oxen?

Neat, but too complex for uncommon.