Ooh, good question!
If I were making a movie, the capaill uisce would probably end up looking a lot like normal horses for logistical reasons. There are some characteristics (like the ears) in the books that could indicate a slightly more fanciful look, but CGI or prosthetics might make things too complicated and/or weird? Some CGI would be needed, of course, but I’d want to use as many real horses as possible (at least for the ones that interact with humans) to cut costs and foster believability.
Marwari horses and Akhal-Teke horses have a definite sea-creature-look but acquiring enough of them for production might pose a problem? Multiple horses are usually needed to play just one character (I think there were 13 or 14 Joeys in War Horse). So ultimately, I’d pick Thoroughbreds or Thoroughbred crosses for their speed and stature (and Corr has always looked like a muscular thoroughbred in my mind anyway…or perhaps a Thoroughbred/Friesian cross).
But if I could find a breed that produced fast horses who were brilliant at acting and looked like muscular 17hh Akhal-Tekes with Marwari ears and feathers, I’d go with that! :)
This also makes me wonder what movie!Dove would look like. I usually imagine the island ponies as looking like Connemara ponies, but I could see Dove as a Welsh Cob based on height and jumping ability. What do you all think? What are your picks?