I'm not sure, but I may have owned a horsehide A2 jacket once. I was about 14, and found a leather jacket at an army surplus store that, as far as I was concerned, looked enough like an Indiana Jones jacket. This was 1982, when you couldn't log on and buy a screen accurate jacket from England. I found a brown leather jacket with cargo pockets, and I jumped on it like a bum on a bologna sandwich.
Anyway, I didn't know what it was made of, but it was very stiff and heavy. Very, very heavy. I out-grew it within a couple of years and went back to the same store to get a new one. The new one was much softer and a slightly darker shade of brown. I didn't know why it was different at the time, but knowing what I know now, I'm guessing the first A2 was horsehide, and the second was cowhide. Unfortunately, I have neither now.
From what I've learned about horsehide, it was the leather used in the original A2's from WWII. But because it was so heavy and stiff, eventually goatskin became the leather of choice. If you are interested in pre-aging your jacket, it won't work with horsehide. MK explains all this in his Regular Guy website.
I've had a goatskin A2 since 1988, and its been through some crazy stuff over the years. The sleeve cuffs are kinda torn up, but the leather is in excellent shape. And I only just recently found out you're supposed to TREAT leather from time to time.
If you'd like a tough leather that will still look good years from now, I'd go with goat.
Oh, and jpenman, the razor cut in my jacket involved more tears than laughter. Suffice it to say, never show your idiot friend your brand new jacket when he's holding a sharp object.
It's like telling a lion your wearing meat pants.