Snowy is Tintin's most faithful partner, his dog. Snowy is a small terrier-like creature with tufted ears (someone better versed in canine species than I can probably identify him - a Jack Russell perhaps?). Quite smart, Snowy is often shown conversing with Tintin in classic dog sense, where they're both talking but Tintin doesn't understand what Snowy's saying precisely.
To us humans, Snowy says "Woah!" a lot, which I presume is Belgian onomotopoeia for a bark.