If you're browsing through horse ads you may see "HYPP negative" or "HYPP N/N" (Or N/H).
HYPP is Hyperkalemic periodic paralysis. It's also called Impressive syndrome.
Impressive was a Quarter Horse stallion who lived from 1969 to 1995. He started a new trend in halter Quarter Horses, for heavily-muscled horses, and was the first World Champion Open Aged Halter Stallion in his breed.
He sired over 2,000 foals.
Unfortunately, he also carried a genetic defect - although he himself showed no symptoms, the condition cropped up in his foals and most obviously in horses that had been linebred back to Impressive. Because he sired so many foals, the condition is fairly common in Quarter Horses, Appaloosas, and Paints (and very rare outside them).
HYPP causes "episodes" of muscle trembling and weakness in the hind end. The horse may even go down, the voice changes and the third eyelid flickers across the eye. Because of the high risk of falling, horses with severe HYPP are not ridable, and it's also believed that they are in significant pain.
The AQHA has attempted to eliminate the disease by requiring testing and refusing to register homozygous (H/H) horses. The Appaloosa and Paint societies, however, have not yet followed suit - and many breeders of heavily muscled halter horses (which are not ridden) continue to ignore or even favor horses with HYPP.
If worldbuilding, consider how a defect like this showing up in particular lines might be treated in your world. Humans are notorious for breeding for traits that have downsides, even major ones.