I just found it funny that horses will be afraid of camels. Elephants, I can understand why... My friend said that it was because of their size, as according to him camels are taller than horses. I think that's BS.
There's a huge difference in specs between four computers and Nagato!
Horses in general are pretty freaked out by new things they've never seen before. So when they see large creatures, moving in a funny manner, making strange noises and emitting a very strange smell they've never experienced before, they get panicy.
Until they get used to a tank they would shy away from it. When automobiles were first sold in the US some states had laws where if the auto spooked a team of horses he would have to hide it in the bushes till the horses passed. They couldn't have freight transport of supplies between areas being upset by a few noisy automobiles. Check maybe your state still has these laws on its books.