Horses often are tight and/or have trigger points within the superficial glute muscle. Here is something you can do to help work that knot out before you ride.
Use direct pressure down on the point. You can use your thumb or a finger (or multiple if it is a large spot or if the one localized pressure is too painful).
Wait for the release. If there is not a release right away, that is fine. If there is no outward sign of a release (licking/chewing, deep breath, flatulence, etc), that is fine. Give it at least 30 seconds. In this video, the mare drops her head and is agitated before she releases. If the horse becomes a bit agitated, but not acting painful, keep up with it and see if you can get that release.
Next, press and rub 90-degrees to the muscle fiber (cross fiber friction) to help separate the sticky muscle fibers.
The last step is to use a bit of compression along the length of the muscle. This is the step the horse usually enjoys the most.
The horse may release at any and all of the three steps.