Look all you want, everywhere you can, and there are still some things that '"everybody knows" that you will never find in official documents, specifications and the like.
I'd be amazed if you can find any official requirement or request stating the round must be capable of taking down a horse. Because it wasn't the kind of thing they wrote down in those days. You will find phrases like "adequate for military needs" and things like that, and you will find the Army's requirement that the new pistol be in .45 caliber...
what you won't find, is a requirement that says "be a .45 caliber so it will drop a horse"....
And, before we get too far down the road of thinking it was to drop the enemy's horse, or that pistols were the intended weapon of choice for stopping horses, lets consider a couple of points that 21st century man usually doesn't consider..
First, it wasn't JUST the enemy's horse that was the concern. It was YOUR horse (and those on your side) as well. Everyone was horse mounted, the pistol is a tool, most likely to be on hand (or on your belt) when needed. Powerful enough to cleanly put down a horse was a personal safety matter, AS WELL as one for combat. AND, while shooting the enemy's horse does remove most of his mobility, there is also wisdom in the saying "a dead horse gives them cover, a live horse is a whole lot of scared unpredictable.."
Also remember that when they were looking at the new auto pistol, the Army had long experience with the .45 caliber (and black powder), and a very recent BAD experience (in the Philippines) with the .38 caliber.
What the Army accepted, which became the 1911, was a semi auto 7 shot pistol delivering the power of the .45 Schofield round, and, while less powerful than the .45 Colt, was a known and accepted value, and "suitable for military use".