For weaving, the hairs are used "as is" (ie not spun). This means the maximum width of the web is determined by the length of the fibres.
Historically, it is not uncommon (in Sweden) to find upholstery fabrics of horsehair from the early 1800s and forward. Horsehair fabrics are very hard-wearing. The fabrics were woven both patterned and plain.
Here is an article (in Swedish, but it has pictures) about a fabric found in Smålands museum.
One of the few European mills that still weave industrially with horsehair is John Boyd Textiles in England.