The Messenger of Allah ﷺ forbade sacrificing an animal whose horn is broken. I mentioned this to Saeed bin Musayab, who said, "Yes, but only when half or more of the horn is broken off."
أخبرنا حميد بن مسعدة عن سفيان وهو ابن حبيب عن شعبة عن قتادة عن جري بن كليب قال سمعت
نهى رسول الله ﷺ أن يضحى بأعضب القرن فذكرت ذلك لسعيد بن المسيب قال نعم إلا عضب النصف وأكثر من ذلك
[Its chain of transmission is hasan] Abu Dawud narrated it in the Book of Sacrifices, Chapter on What is Disliked in Sacrificial Animals, no. 2805, from the hadith of Qatada, through the same route, and Tirmidhi said in no. 1504, hasan sahih, and it is also in al-Kubra, no. 4467.