The snow ant is identical to the fire ant in terms of stats, save for its elemental affinity to cold - as such, it conveys cold resistance, and is not infravisible. Its cold attacks have a chance to destroy potions as usual, subject to magic cancellation.
Snow ants are randomly generated in normal levels on the same basis as fire ants, with the exception of not appearing in Gehennom due to being icy monsters.
The techniques for disposing of other ants also apply to the snow ant: make judicious use of Elbereth, attack wands and ranged attacks and try to fight them in a corridor or (preferably) near the up staircase. In particular, don't be afraid to run away or use escape items.