Hujayyah bin Adi narrates from Sayyiduna Ali (may Allah be pleased with him) that he was asked about the cow, and he said: "It is on behalf of seven people." And when he was asked about an animal with a broken horn, he said: "There is no harm." The man asked about lameness, and he said: "When the sacrificial animal reaches the place of sacrifice (then it is fine); the Messenger of Allah (peace and blessings be upon him) commanded us to look carefully at the ear and the eye."
Imam al-Shafi'i (may Allah have mercy on him) says: The horn being broken or absent does not harm the sacrificial animal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19107