For some interesting history on animal behavior check this out. https://en.m.wik...

dmckeon · on Feb 14, 2016

"One person presents the photo while another holds the horse,"

Indeed. One wonders if the person holding the horse and the person presenting the face photos could see if the photo was of an angry or other face.

Phemist · on Feb 14, 2016

From the article:

    Experimenters were blind to the stimulus
    in 82% of trials and analyses showed no 
    difference in subject responses according
    to this factor (see the electronic supplementary
    material tables SII and SIII).

So, it didn't even matter. Pretty much everyone that would be doing research of this sort is familiar with Clever Hans, so I'm sure they'd be extra careful when designing their experiment.