Ibn Jurayj (may Allah have mercy on him), narrating this hadith from Ata (may Allah have mercy on him), asked him what "mukafa'atan" means. He said: "That the two are alike." And a sheep is more beloved than a goat, and a male more than a female; this is his own opinion. Someone said to Ata (may Allah have mercy on him): What if I slaughter a camel in its place? He said: Begin with the one that has been named, and after that whatever you like. I said to him: Is this the Sunnah way? He said: Yes, this is the Sunnah way.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19279