Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both), concerning the verse: "Eight kinds: of sheep two (i.e. male and female) and of goats two (i.e. male and female)" [Surah al-An'am: 143], says: The eight kinds are the pairs, i.e. male and female, of camels, cattle, sheep and goats, according to what is easily available, and those that are bigger are better.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الضحايا / 19083