First of all I'm very sorry for your loss. The fact that it was sudden makes it even worse. Not that time to prepare makes the death of a beloved pet much easier just less of a shock.
I think a weeping willow would be safe for your other horses. I found this - No reference to human injury found in any reference. Toxicity rated as ‘none’ for the species summarised in the USDA Plants Database (2006) including S. purpurea, S. exigua, S. nigra, S. x sepulcralis, S. alba. Presume no toxicity for any Salix.
The only thing I found that listed potential harm by this tree is geared toward fish - The first is that its all pervasive and vigorous root system may disrupt the pond. The other is its foliage, which contains a substance that is akin to aspirin. This can kill fish if present in sufficient quantities in the pond. It is released as fallen leaves decompose in the water.
Hope this helps.