Winter's Mount

 
Prerequisite(s): Special mount ability.
 
Benefit: Your special mount gains the cold subtype. It gains immunity to cold, but has vulnerability to fire, which means it takes half again as much (+50%) damage as normal from fire, regardless of whether a saving throw is allowed, or if the save is a success or failure. Your special mount is also hardier than a nonfrostfell version; it gains a +2 racial bonus to Constitution.
 
Source: Frostburn