Equine reflex hypertonia, commonly called stringhalt, is a neuromuscular condition that affects horses, causing the hind leg to jerk up, folding up towards the abdomen, and sometimes accompanied by a strong stomp back to the ground afterwards. This may be mild enough that a person not familiar with horses might not notice the difference, or may mistake it for a light cramp; it may also be quite severe, sometimes even leading to the horse hurting itself.
Generally, the affected leg will jump up strongly with every step, often most strongly when turning, walking backwards, or going down a slope; it often becomes reduced when the horse moves into a trot, and may be absent during a canter. Stringhalt is a simple muscle spasm, and is generally not accompanied by pain. It does not generally make the horse unfit for most horsely activities.
Stringhalt may be due to muscle injury or the accumulation of toxins from eating certain plants. When caused by plant toxin, it is usually referred to as acquired stringhalt or Australian stringhalt, although it is more technically referred to as plant-associated or pasture-associated stringhalt. The most common culprits are flatweed (a.k.a. false dandelion, Hypochaeris radicata), sheep's sorrel (Rumex acetosella), and couch grass (Elymus repens), although there is currently some debate about the role of fungal mycotoxins in stringhalt. Depending on the amount of toxin the horse has consumed,
stringhalt might also be present in the forelimbs and even the neck, including the larynx, which may result in the horse's voice changing; toxin-based stringhalt is also much more likely to be present bilaterally in the rear legs. Acquired stringhalt is often temporary or seasonal, and is best treated by removing the horse from the pasture containing the toxic plants.
Those cases of stringhalt that are not the result of mild self-poisoning are generally termed idiopathic stringhalt, although it appears that it may be due to muscle injury or, in severe cases, abnormalities of the thalamus. While there are treatments -- including surgery and medications such as Botox -- these are not reliable. It appears that damage to the nerves travelling from the brain to the lateral digital extensor (a very long muscle in the leg) are the most likely cause, and it is most often a type of distal axonopathy. It's harder for the body to repair distal portions of nerve cells, so damage from the toxins show up in the part of the body furthest from the axons -- in this case, the rear legs.
If you have charge of a horse with stringhalt, you should probably check in with a vet, not me. However, cold weather, hard exercise, and anxiety/excitement can make acute symptoms worse. There is nothing that makes stringhalt better in the short term, and no certain treatment for the long term; it's a case of managing symptoms and hoping -- or, in many cases, simply accepting that this is a harmless condition, and your horse just walks funny now.