I have heard more about worm castings being acceptable for aquarium use, but mostly not larger animal waste.
I add worm casings to my MTS to provide some organic content along with Oak Leaf mold. Forum Member Larcat also uses worm casings a log with a few others that post here.
A horse in a stall may have any of many different materials as bedding. (around here, rice hulls and pine shavings are most common)
He urinates in the bedding. He poops on it. He spills water and food onto it.
Then the wet bedding, poop and wasted food (hay) are tossed in a pile to compost.
The high ammonia content (urine) plus the high carbon content (bedding) make this blend compost really fast. Turning it with a tractor, you might get garden ready compost in just a couple of weeks. I think I would compost it longer for possible aquarium use.
Straight manure, perhaps with a little bedding stuck to it, or maybe collected from runs with no bedding will have almost no urine, so the ammonia content is lower. Not zero, though. Horse digestion is not very efficient, and if they are fed high protein foods (alfalfa, grains) there will be plenty of ammonia to blend with the high carbon (dried grass) part of the manure.
The issue with stall bedding I mentioned has nothing to do with your observations and postulations.
It has to do with a study that found wood shavings compete with organic materiel for N, causing the the wood shaving / horse manure to be less effective for agriculture use than other compost sources.
I have the source quoted on Post#72 on my Toxic Ten Journal thread. I believe I've also cited studies that grade and rank POOP used in compost. Lamas, if I remember correctly were the highest ranked.