I have always assumed the horses are Clydesdales, but I really don't know for sure.
I'd love to hear from someone who can do a positive identification.
Continuing the theme of Clydesdale Horses here is the song "Heavy Horses" written by Ian Anderson and performed by Jethro Tull
Heavy Horses, move the land under me
Behind the plough gliding --- slipping and sliding free
Now you're down to the few
And there's no work to do
The tractor's on its way.