I went to our local leather shop and found a good deer hide to put on a banjo. It tightened in well and is very resonant. It also was about half the price of the calfskin heads that Stewart McDonald had in their catalog.

The only problem with some deer hides is they have scars on their backs from crawling through barbed wire fences.

I imagine goat skin would do as well (they use them for bodhrans don't they ?!?). The only horse hide I ever saw was pretty thick, probably have to do a lot of thinning down before you used it.