I may be the last person to find this, but...

As most people are probably aware, there are a number of keybindings that select predefined groups of units when you're a general.  "H" selects your Demigod, "M" selects all your minions, "N" selects your Demigod, and "8", "9", and "0" select minotaurs, archers, and priests respectively.  I thought for a long time that it was an odd oversight not to have a key that selected just the special minions (spirits/night walkers/yetis/shamblers), until today I pressed "I" accidentally and they got selected!  So, I guess there was no oversight after all .