The horses in Skyrim are Shire horses. Biggest horse breed in the world, built for Skyrim's geography and weather. RDR is just a normal mid-size horse breed.
EDIT: http://i.dailymail.co.uk/i/pix/2009/11/15/article-1228089-013CD30400000578-445_468x293.jpg
EDIT 2: http://www.republicdomain.com/photos/bulkupload//wallpapers279/Valley-Horses.jpg
I prefer Skyrim honestly. Beautiful creatures.
I don't think they are Shire horses, there might possibly be some in game but shire horses arn't typically used for riding more for pulling things and I think you'd have a hard time getting a shire horse to move as fast as the do in Skyrim. plus Shire's are massive.
I'd say they are more like Cob's and Clydesdale's, whereas RDR had something more like quarter horses and thourughbreds (sp?)
It makes sens that they are more stiff after all Cob's and Clydesdales are the right type of horses for the Skyrim enviroment, hardy and strong
We own horses so I know a bit about breeds and stuff but I just think Shire horses would be too big and cumbersome to be a mount in Skyrim