At the end, each student was given an individualized analysis of their personality based on their test responses and asked to say how well this described his/her personality. The test was judged to be "very accurate" by most of the participants. In fact, everyone was given the same analysis regardless of how they answered the questions! (This is called the Forer effect after the psychologist who devised the experiment.)
We do this with our first years
Two things spring to mind for me when looking at the "horsenality" thing. First, personality (which looks exclusively at humans), is considered to be stable across the lifespan - in other words, if I met you when you were 12, and then not again till you were 35, your personality is the thing that makes you uniquely
you, from my point of view

So the "horsenality" actually sounds like a classification of how your horse is behaving, and how you can work to change that, rather than a description of a horse equivalent of personality
Second, there has actually been quite a lot of academic research on the equivalent of personality in animals - you won't find it by searching for "horse personality", because it has always been referred to as temperament
Humans are considered to have 5 "independent" aspects of personality - so you can vary on one without it affecting any of the others. They are Open to Experience vs Conventional Interests, Outgoing (extravert) vs prefer peace and quiet (introvert), Agreeable (gets on well with others) vs Reserved (more suspicious of others), Emotionally reactive (called Neurotic) vs Calm and stable, and Conscientious (preferring plans and goals) vs spontaneous and unplanned.
Three of these 5 are considered to be shared by many animals, including horses - they're the Extravert/Introvert one, The Neurotic/Stable one and the Agreeable one. So you should be able to rate your horse's personality -rather than just their current behaviour - on these three. My horse, for example, on a 1 to 5, would score as very highly extravert, very highly agreeable, but on the high side for Neurotic

His favourite mare would score low on Extravert (i.e. introvert), but high on Agreeable, and similar to mine on Neuroticism

The lead mare in the field is introverted, but highly agreeable and low on Neuroticism.
So there we go, a system for rating horse's temperament, based on research (the key author is called Gosling

) and which is informative about how horses differ from each other. And you don't have to pay anybody to find out about it
