Because there used to be 2 of them, but it's offensive to joke about that now.
Pair-o-Medics
Person 1 : Suggest me a good phone to buy nowadays. Person 2 : Microsoft Lumia 950 XL is good for winters, will keep you warm. Very warm. Person 1 : So what about summers then? Person 2 : Same, it freezes often as well
One they've never herbivore.
Miscarriage. This joke never gets old, just like the baby.