Have you done any obedience training with the dog? Taught a solid "off" or "leave it" command? It is entirely possible to train the dog not to chase, nip at, etc the horse but it is going to take some work. I like to start with the horse loose and the dog on a leash. The moment the dog even THINKS about making a move on the horse I use a correction and give a verbal command (I use "leave it"). Once we have the impulse under control on the short leash we move to a long line (I use our lunge line) and repeat as necessary. We don't move to off-leash work until/unless I feel we are 100% solid on lead. None of this work is done before I have already laid a solid foundation of training in other areas - I consider this to be more of a "finesse" training - sort of doing all your foundation training on a horse before you start asking for dressage work. Once we have it under control on the ground, we move to generalizing the training to include when I am on the horse.
It is also important to have solid training for your dog before you start riding as you need to be able to have total control over the dog from the saddle -- not only in relation to how the dog is interacting with your horse, but also how you control the dog in regards to the environment around you, keeping him/her from taking off after wildlife, keeping him/her from fighting with other dogs you might encounter, etc. For the dogs I work with "leave it" is taught to mean that is not yours and you are not to touch it, ever -- "off" is used as an attention getting, stop doing that sort of command (ie if they are chasing something and I want to recall them - off gets their attention, they immediately break off of what they are doing and I can then issue the recall command to get them to return to me. It is taught as a sharp, single syllable LISTEN TO ME NOW sort of association for them to break through the heat of the moment deafness that even the best of dogs can fall into when they are intent on chasing something, etc.
Last edited by themacpack; 10-20-2012 at 08:45 PM.