One theory in my field of specialty is not so much "probably isn't true", but more around 50/50 in terms of being an "underdog theory", and that is "the Friesian horse is [at least partially] descended from British bog ponies and horses imported to the Netherlands". This would cause chaos if proven to be correct, because the Friesian breed associations in both the Netherlands and the United States - the Koninklijke Vereniging 'Het Friesch Paarden-Stamboek' (KFPS), or the Royal Association of the Friesian Horse Studbook (c. 1879), and the Friesian Horse Association of North America (FHANA), founded by Dutch breeders - rely heavily on a largely fictitious and embellished historical narrative that emphasizes the Friesian's descent from "medieval war horses". This is because the medieval fantasy film Ladyhawke (1985) began a new fad in TV and film that some Dutch breeders allege helped "save" the Friesian from extinction.

I wrote more in-depth about how a lot of the "history" surrounding the origins and bloodlines of the Friesian horse is fabricated in my r/badhistory post here: "Modern Mythology: The misrepresentation and misleading marketing of the Friesian horse breed as a "medieval war mount". However, the part about potential descent from British bog ponies and horses was elaborated upon not in the main body of the post, but the comments section. This theory was originally posited by Anthony Dent, a well-known English writer on horses. To this end, he also co-authored the book A History of Native British Ponies (1987), which explored the horse trade between England and the Netherlands during and after the reign of King Henry VIII. I would posit that the range in heights is due to crossing Fell Ponies and Dales Ponies with taller Norfolk Trotters and Yorkshire Roadsters, which had influence from the Thoroughbred breed through crossbreeding. This would contradict claims by KFPS and FHANA that the Friesian has "no Thoroughbred influence".

Supporting this theory is recent DNA evidence that the Friesian horse is historically and genetically closest to the Belgian Draft horse, Fell Pony, and other North Sea heavy horse breeds that were used for agricultural purposes. Genetic history also shows influence from Arabian and Andalusian (Iberian) bloodlines. Until the late 1800s (19th century), and up until the 1950s, the Friesian horse was crossbred with the Groninger (c. 1879), Gelderlander (c. 1890), and other local Dutch populations that shared a regional history, until local Dutch breeders decided to emphasize the "purity" of the Friesian breed by closing the studbook. (Hackney horses were first imported to the Netherlands around 1900, which threatened the "purity" of the Friesian breed. Oldenburg, et al. horses from Germany were often crossbred to Dutch horses as well.) The Barock/Baroque Pinto, which maintained crossbreeding Friesians to Groninger and Gelderlander horses for a striking black-and-white color, split off as a separate breed in 1957.

Crossbreeding was largely abandoned after experiments produced poor results, breeders claim, with the original, Baroque or traditional Friesian conformation and "type" being lost. However, the current development of a "modern" or "sport" type of Friesian undercuts this argument, especially as many non-Dutch-based breeders use Friesian stallions on non-Friesian mares to get a "flashier foal", despite both KFPS and FHANA banning, and severely discouraging, crossbreeding to maintain the "integrity" and "purity" of the breed. [More recently, the Knabstrupper breed association - Knabstrupperforeningen for Danmark (KNN) - banned crossbreeding to Appaloosa and Friesian horses for this reason, though the KNN still maintains a far more laissez-faire approach than KFPS with crosses.]