Soring is a controversial practice that involves using caustic substances to "enhance" the gait of a gaited horse. It's most often seen with Tennessee Walkers (which has sadly given the entire community surrounding the breed a bad reputation, even though the majority of TWH owners aren't involved).
Soring is also sometimes done by "pressure shoeing", which means nailing a shoe tightly onto a closely trimmed food. (Shoeing is not normally painful for horses, although some do show signs of being annoyed by it).
Soring is illegal in the US, but the law is poorly enforced. Fortunately, the movement against it is strong and the practice is becoming rarer all the time.
A Tennessee Walking Horse performing a natural gait - image source Kersti Nebelsiek via Wikimedia Commons.