Has anyone ever flown as an attendant for their own horse while flying back from Europe?
Yes. Multiple times.
What company let you do so?
I find KLM to be the most horse knowledgeable airline. They insisted that we be with the horses on takeoff and landing. Every other company I’ve used lets you fly with them, but might not let you see them until after takeoff. Just tell your broker that you want to fly with a company that lets you accompany the horse.
It is required that 1 attendant fly per box, so just let the import broker know when you book that you want to fly. They’ll arrange the 1 way ticket for you. You will have to have your passport book and be able to pass a background check.