They all appear to be Lusitanos or of Iberian breeding. (I can tell for sure on the first few because of the rider's apparel, he looks suited up for Equitacao de trabalho.)
The first couple are Andalucians. Come to think of it, I don't really see a lot of Lusitanos with that much flourish in their hair? Maybe because they were more used for bullfighting. The bottom two look more like Peruvians to me. The middle historical photos actually look like Morgans or Saddle Horses?
Part of it is the breed/genes...Iberian, or otherwise Spanish descent, and Baroque horses, as well as most of the "Romantic breeds" are all known for HAIR. Like the beloved Anton. Frankly, I think when it hits a certain length it looks skanky. It's just TOO gosh darn long! Like those woman whose finger nails are so long they curl under? Why??
As for upkeep and reaching that length, it's daily maintenece of a stall kept horse. Braided and bagged everyday. Wash every 3 to 7 days, then re-braid and bag. Spray the braided mane daily with water to keep it moist and lessen any irritation and avoiding rubbing. You don't wanna brush it daily, either. I really don't think there's any other way. Personally, I couldn't keep a horse like that...I want them to be horses, yah know? Sane and happy and healthy.
Here's a picture of my Friesian TJ with his hair at it's longest. It was long enough for me, let me tell yah! He also shares good genes with Anton as Oege was his grandfather on his mom's side!
And for a horse with so much hair, he hates having it fussed over! Such a brat!