For a lamb to have horns is impossible in nature, let alone to have seven of them. In the Bible horns are a symbol of power. The number “seven” denotes fullness. The combination means that the triumphant, ascended Lamb (= Jesus) has fullness of power (Matthew 28:19).
6 And between the throne and the four living creatures and among the elders I saw a Lamb standing, as though it had been slain, with seven horns and with seven eyes, which are the seven spirits of God sent out into all the earth.