As far as I can tell this would work actually. I can't see anything in the rules text that says it wouldn't. The creatures would certainly die on impact (excess damage in this case will carry over to their normal form's HP pool) but they were fine being summoned into battle to possibly die anyway so I don't see what changes, they'd probably still be considered willing considering they are summoned creatures under control of the caster. And the spell states they retain the transmuted form for the duration (24h) or until dropped to zero hit points, and does not state the creatures would revert when leaving the spell area.

Well fuck me. I don't want that to work, but I think it does. I can't argue a reason it wouldn't. Have fun carpet bombing your enemies with horses, I guess.