I think its possible... But it is kinda complex i think.
I dont know the exact steps - so I apologise I can help you fully. However I thought of a couple of things that may be useful.
First of all, your going to need a model of yourself riding a horse. You the have to animate it.
This is a tutorial on how to animate a model:
http://wiki.blender.org/index.php/Doc:T ... _Animation
This is HOW you should animate it and write a .cfg for the model:
http://sauerbraten.org/docs/models.html
Then you should replace one of the animated characters that come with Sandbox with your model
This spell that you wish to create is going to need to have an effect with it, i.e. when you write the script for it, it will need to have an /playermodel X, in it, where X is the number that corresponds to your character.
Ok, its pretty complex. If you want I can help you with your project, but only if you can get the animated horse model first. Without that (and believe me it takes a long time to make - unless you can find one) you will never get this working.
Hope I helped,
Owen.