It is not so easy to make them ejaculate.
I have known few horses that ejaculate with oral or masturbation, I only knew 1, 90% of horses need either artificial vagina or put it in some anus or vagina.
The reason is because in the glans they have nerves that detect heat, humidity and pressure, they use them to know if they managed to put it inside their cock when riding the mares.
If there are missing any of those factors, you will just push and squirt all the time.
Reguards