I would spend the next month training your mini to accept gunfire, from behind, and carry a weapon. There is no going back if one of those dogs attacks your horse. At LEAST train for and carry a beebee gun bc you can shoot a dog with one of those and not kill it. You will NOT able to get off of the cart and reach your mini in time to help with pepper spray.
Before I got my dog, "Rose" (b. May 8, 2007, Husky/GSxBC mix), I wanted a companion for my 8yo, docile GS/Collie cross ("Xena," 1998-2009, RIP) I went to a local shelter and got a 7mo GS (on a Friday afternoon). Gorgeous dog. I thought I got another Xena bc he was very friendly to people. Xena sulked when we shot possums in the barn and tried to protect several baby Robins that would fall out of their Spring nests.
He tried to kill two of my cats and attacked one of my full sized horses on the leg, unprovoked. He went back the next Monday.
Food for thought.