When Jeor Mormont tells Tyrion Lannister that some of the wildlings that the Night's Watch have captured claim to have seen the White Walkers, Tyrion dismisses this by saying that sailors at Lannisport claim to have seen mermaids.
In the books
In the A Song of Ice and Fire novels, mermaids are dismissed as legendary creatures that don't exist in real life. Sometimes drunken love-sick sailors who have been at sea too long report seeing them, but these always turn out to be sea cows, etc.
Animals and Plants of the Known World
|Legendary and magical creatures:|