It means Beware of the rooster! Are you sure? Maybe you made confusion, we don't have dangerous roosters... I think you saw "Attenti al gatto", "Beware of the cat", a joking warning from cat's owner, often you can read this on the rug before main door.

"Attenti al gallo" - mi venne in mente i film francesi della aventure di Asterix e Obelix con Depardieu:)

Some roosters may attack people so if you saw the sign in the country the phrase might make sense. For dogs the standard phrase is "attenti al cane"