[quote=The Imagination]
Irradiated horses in the midwest, anyone? Sleipnirs?
[/quote]

What about Bison?

And I think in an official Fallout comic they showed the NCR had a population of healthy non-irradiated horses they use in their army.

Actually...

[img]http://static3.wikia.nocookie.net/__cb20101227035027/fallout/images/9/98/FNVAR_Horses.jpg[/img]

From "Headed West".