Something about the experiment design seems off, but I can't place exactly what.
I don't know if it is the same thing you're talking about, but I thought it was interesting that they could assume that the horses would know that a blanket causes both relief from cold weather and overheating in warm weather, and also that they should think of the blanket based on how they were feeling. I thought those assumptions were almost as interesting as the part about symbols because of what they seem to imply about the horses' ability to reason through cause and effect.