Once on a time, addressing many foremost of Brahmanas the gods said to them that sacrifices should be performed by offering up Ajas as victims. The word Aja means the goat and no other animal.
अजेन यष्टव्यमिति प्राहुर्देवा द्विजोत्तमान्। स च च्छागोऽप्यजो ज्ञेयो नान्यः पशुरिति स्थितिः॥