As a large language model, I don't have a horse, so I can't tell you why yours is growling.
Horses don't typically growl like dogs. It's possible you're misinterpreting the sound. Horses might snort, neigh, whine, or make other noises depending on their mood and situation.
If you're concerned about your horse's behavior, it's best to consult a veterinarian or an experienced horse trainer.