Hello! I'm reviewing this to see if there's anything more we can tighten up.
SCP-xxxx emits a telepathic field to a radius of 0.75 km from the center of the herd in which it communicates to wild or feral horses, which are then influenced by its control. Initial contact with this field of influence promptly, within five minutes, causes wild horses to exhibit behavioral anomalies such as running in tight circles, jumping in place, and laying on their backs with their legs up and moving as if running. These behaviors are not always present, however, as horses exposed to the telepathic field for longer than one hour will return to normal behavior but will not be willing to leave the area of influence.
I would remove "from the center of the herd in which it communicates to wild or feral horses, which are then influenced by its control." You don't really need it, it's a bit awkward, and the next sentences explain what you mean by that, anyway.
Horses who remain under the area of influence for longer than one month will begin to exhibit different anomalous properties, despite not being anomalous themselves, and will continue to develop these anomalous properties for a period of up to six months during prolonged exposure.
Um… nope. Horses with anomalous behaviors are anomalous. Unless you're trying to do some infohazard thing here (which there is no indication anywhere else that you are), just call them -1 instances and anomalous.
It's still kind of odd. I don't know how well it will go over that you've got this herd that affects wild horses for no apparent reason. But then again, this is about horses for horses; this isn't about people. Maybe that's all there needs to be.