All the horses died. Luckily a tour helicopter was able to relay to the hikers that a 40ft wall of water was coming and their "high ground" wasn't going to cut it
http://blog.photographersadventureclub.com/trapped-trail-havasupai-due-flash-flooding/
The helicopter pilot flew about 50 feet around a bend to tend to the handlers and horses. Luckily, the handlers were all airlifted out – but the horses didn’t stand a chance. We saw why later: the walls around that bend were sheer – from the canyon floor to the top – there was no path or ledge to make it to higher ground. With the water rising that rapidly there was absolutely nothing anyone could have done. I am glad that circumstances had slowed us enough that we hadn’t yet reached that point in the trail.
New link for Chris: http://www.myfoxphoenix.com/story/23398536/2013/09/10/mesa-man-survives-flash-flood-on-trail-to-supai