How to relieve a stiff horse ?

I am a certified Equi-Bow Practitioner and use this non-invasive, gentle touch technique to not only relieve stiff muscles, but help prevent them by creating a balanced horse. Prevention is the key to fewer injuries/pain. EQCentric.com
Do you know the cause of the stiffness? Professional equine massage and stretching will absolutely help reduce stiffness but it is important to find the root cause so that you can prevent it as best as possible. Feel free to send me a message. I deal with this professionally.
stretches, massage, liniment, chiropractic, gymnastic exercises, rest. any or all of those could be applied, but youre going to want to try and find the source of the stiffness if it happens regularly. could be saddle fit or how theyre ridden. some horses it comes down to their conformation or if theyre developing issues like arthritis.
It is necessary to promote the evacuation of toxins from the muscles of the horse, for example by performing a small trotting of five to ten minutes, just after the effort (active recovery).
Then, a good shower to facilitate the drainage of the limbs and, why not, the laying of rest bands or refreshing gels.
One or two days later : priority to quiet exits ;)

You also have some products of Ravene : Nutrylite solution, Nutylite Flash or Tendiflex @ravene :)
To relieve stiffness, you can stretch it and massage. The best still is to leave to rest the following day or do a moderate activity (walks, trottings ...)
Maybe linament or a professional massage. Forgive me I've never had a stiff horse so just spitballing here.