Do specific mobs drop the Whimsyshire legendaries?

So my wife and I have started accumulating our machine/act 1 key run NV stacks in Whimsyshire in the hopes of occasionally picking up a 200+ dps Hamburger or Spectrum.

Yesterday, we both got Spectrum on the exact same drop from a particular purple mob--Team Unicorn. They're four of tiny unicorns with names like R'lyeh and Sorely Maulin. This leads me to suspect that Spectrum drops only and perhaps always from this particular mob. Any info on this?

More to the point, is there a specific mob that drops the hamburger?