It’s possible to see Diablo 2’s Secret Cows in Whimsyshire, thanks to a Cursed Chest. Site reader WickedYeti sent in an screenshot of his battle, when he popped the Diablo 2 style Secret Cows from a red chest in Diablo 3’s Whimsyshire.
Note that this is the actual Whimsyshire, not just that tile set showing up in a Nephalem Rift, which happens fairly often, though seldom with the Whimsyshire monsters attached to it. (You can also get the Whimsyshire monsters in other tile sets in a Rift, and the Killer Teddies and Happy Flowers and Chubby Sparkleponies always look funny prancing around normal dungeon levels.)
Anyone else seen this in Reaper? I haven’t even been into Whimsyshire so far in Reaper of Souls, though now I’m tempted, if only to farm for Cursed Chests while I’m there.