1. Get a companion pet you don't have.
2. use it.
3. put the spell to summon it as a companion pet on your action bar.
4. cage the pet.
5. send it to another char/sell it
6. you can still click the spell on your action bars to summon the pet you once had!
(the spell to summon Darkmoon Rabbit as a companion is on "1", even though it does not have the same image as Darkmoon Rabbit that you see listed in the Battle Pet tab)
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(if you can't get it to work, try using the companion pet on one char, log another char and place the "summon that companion pet" spell on your actionbar, log back onto the first char, and cage the pet)