Voted No...
I have no issue with mounts, but it seems strange to re-introduce a species that hasn't been in the series thus far...
That being said, I really wanted to find one of the children's toys that we saw advertised in Fallout 3 load screens... And mounted brahmin/bighorners would be fun to see.
What about all the other new creatures that gets introduced then? Bighorners? There were no Bighorners in the eastern California or Nevada in Fallout 2, not any Cazadors.
There are loads of other creatures aswell. Why are there no albino radscoprions or bloatflies in California? You'd think flies would be everywhere. The same with wasps, there are wasps in the midwest, Fallout Tactics, but nowhere else in USA? NCR's got a two-headed bear on their flag, yet there are no two-headed bears in Fallout, just one-headed bears but they are on the east coast. There are loads of food made of iguana, but no iguanas.
There also seem to be no giant rats on the east coast, nor spore plants or mantises. Are they uncommon, pre-war, in that area.
Now, I would think it'd be weird if there'd be a game taking place near the areas we've already seen and it'd include horses, and people acting like they've always been there. Sure. Even if I think NCR troopers riding horses would be a perfect fit, including them in a later game like it's always been this way but we haven't seen it before would be stupid. That's why the game could take place some place further away from where we've been, and there could be horses. The horses wouldn't be the strangers in this area, you would. Maybe all the horses migrated north because post-war climate got too hot and praires became deserts, and they didn't like that. You come to a place where people are used to horses, just like you come to D.C. and people know about bears. Or Nevada, and people know about Bighorners. Doesn't that make sense? New creatures are included with every new game, but horses are a big no-no?