Allah Almighty says: "Then the compensation is the like of what he killed, from livestock." [Surah al-Ma'idah: 95]
Imam al-Shafi'i (may Allah have mercy on him) said: "And 'the like' (equivalent) means the like in description of the game that he killed."
Ata (may Allah have mercy on him) says: If the muhrim kills a one-eyed or defective animal of game, its ransom may also be given with a one-eyed or defective animal, though a complete ransom is dearer to me. And if he kills the small young of game, a small young goat is to be given for it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحج / 9892