The sabino pattern is nearly as common in Paints as the frame overo and tobiano patterns. The sabino pattern is usually the one called "overo" in South America, so terminology is confusing at best.
The term "sabino" in literal Spanish means pale or speckled, and in Mexico and Argentina this term is used to describe fleabitten grey horses, or other mottled patterns. In Europe, and increasingly in the United States, sabino is used to describe a unique and interesting pattern of white spotting in horses.
Sabino horses usually have four white feet and white legs. The white usually extends up the legs in ragged patches, and then extends onto the horse's body from the belly. The head is usually fairly white, and the eyes are commonly blue.
Many sabino horses have eyes that are partially blue and partially brown. Flecks, patches and roan areas are common on sabinos, in contrast to the frame overos that are usually more crisply marked.
Sabino occurs in a large number of breeds worldwide, including Paints, Thoroughbreds, Clydesdales and many others. The sabino pattern is the usual culprit when spotted foals appear in breeds that frown on them, such as the British pony breeds and the Quarter Horse.
The sabino pattern is also a great imitator, and some of these horses are nearly perfect mimics of tobiano or frame overo. This adds to the confusion of the sabino pattern. When the sabino pattern is minimally expressed, the horse usually has four white socks and a blaze. Usually there is some betrayal of the fact that these are not the usual white marks on horses, due to some ragged edge or narrow and long extension up the leg.
Some sabinos will also have odd white patches on the knee or hock, removed from the main portion of the lower white mark. A few sabinos do have a dark foot or two, although most have four white feet. Minimally marked sabinos are easily confused with truly nonspotted horses.
In the middle range of expression sabino horses are fairly distinctive and are usually difficult to confuse with other patterns. Most have white extending from the belly. Most have roan and flecked areas in addition to white areas. A few, though, will be nearly entirely roan without patches of white. These could be confused with true roan horses, although the facial and leg white usually gives these away, and they do not have dark heads typical of true roans.
Another extreme is the sabino that is patched, but not roaned. These can easily be confused with frame overos, especially if they have a dark foot or two. Most patched sabinos have smaller, more ragged patches than typical of frame overos. In some cases it is impossible to distinguish between horses that are truly sabinos and the frame overos that also happen to have white markings on their feet in addition to the frame overo pattern.
The whitest of the sabinos are nearly or entirely white. Some retain color only on the ears. Others are indeed white all over. One of the whiter ranges of expression includes color on the ears, chest, and tail base. These are the medicine hat Paints of the native tribes from the Great Plains. Most sabinos that are largely white are very speckled and roaned, and some can be confused with Appaloosas.
Some sabinos are quite white and survive, which points to this being entirely different from the frame overo that results in lethal white foals when homozygous. Sabino, by itself, is not associated with lethal white foals.
The sabino pattern is confusing genetically. In many, or most, families it appears to be transmitted as a polygenic trait rather than as a single gene. Many horses appear to transmit it roughly in the percentage that they are themselves white. That is, a sabino medicine hat is likely to produce a higher percentage of spotted foals (or at least foals registerable as spotted) than is a minimally marked sabino.
Breeding for the sabino pattern has a few interesting quirks. In many breeds it is desirable to have flashy white marks, but not body spots. This includes the Clydesdale, Shire, Welsh Pony, Arabian, and even the Quarter Horse (for some breeders).
Clydesdale breeders especially like the white marks, but most prefer a bay body color. The general rule that many Clydesdale breeders use is to mate horses with four white feet (and usually roany bodies, resulting from the sabino pattern) to horses with one dark foot. What this tends to result in is the mating of horses with too much sabino expression to those with too little expression. On average, the resulting foals come out with fairly minimal expression. In the Clydesdale, this means few body spots and relatively few roans, which pleases most breeders and buyers of this breed. They still do get the occasional medicine hat, though.
Some horses that lack body spots, but have the high white socks that creep up toward the body, are indeed sabinos and can be useful in Paint breeding programs. The sabino pattern is a great pretender, but is also responsible for some very attractively marked horses.
The sabino pattern is probably the most common "cropout" from the Quarter Horse and Thoroughbred breeds. In many cases, an investigation of the cropout's parents reveals horses with extensive white markings. These are probably minimally marked sabinos, and occasionally produce foals with more sabino expression, that are therefore recognizable and registerable as Paint spotted horses.
A few instances of cropouts, including nearly white ones, have very dark parents, or even parents with no white marks. These are clearly not sabinos, and these raise the question that there may be mechanisms that can mask the expression of the sabino pattern.
If the sabino pattern is merely an extension of "normal white marks" then this means that an occasional solid-colored horse (with no white marks) may be able to mask both white marks and the sabino pattern. The practical consequence of this is that such horses make poor choices for an outcross breeding program, because they can decrease the percentage of spotted foals.