It's a very good question, because it's a little known disease.
Ataxia is an incoordination of voluntary movements (both at rest and in activity). This disease is very serious, it can range from a stiffness of the limbs with a staggering gait to a partial paralysis with a horse lying on the ground without possibility to rise again. The horse seems to have the hind legs planted in the ground, and it sits if it recedes, then falls. Unfortunately the more the falls are repeated the more the condition of the horse becomes serious, until a fatal issue ...
For the precise definition, ataxia is a compression of the spinal cord, due to a malformation of the cervical vertebrae, and a narrowing of the spinal canal.