A device (1) to tag an animal is disclosed. The device comprises a female (20) including a case (21) with a thorough hole (22), a male (40) to be coupled to the female (20) and including a stem (41) with a tip portion (60); the female (20) includes tamperproof fastening means (23) adapted to block the tip portion (60) in the thorough hole (22) and wherein, after coupling the male (40) with the female (20), the hole (22) is closed by the tip (60), the tip becomes part of the female (20) and the stem (41) moves in the case (21).