Clearly that the sacrificial animal represented the buffalo demon Mahisha, slain in the Devi's martial exploit. The demon represents the evil forces within us. The sacrificial animal may thus represent the demon, who in turn, may represent brutish, ignorant, arrogant, or other negative qualities within us ourselves.
Tyeb Mehta's "Mahisasura"
The blood sacrifice as part of Durga puja, have eliminated altogether from the celebration, while others offer a vegetable substitute in place of the animal. However, the sacrifice is necessarily present at any form and symbol, and it symbolically commands an important place in the ritual. The buffalo is traditionally the favored sacrifice to the goddess, with a special role reserved for the head. In both myth and ritual the buffalo is a symbol of power, which is transferred to the Goddess through sacrifice. In some village rituals, the buffalo’s blood is drunk in order to ingest its strength. What, however, is this ultimate sacrifice that can be made to the Goddess? Clearly, the most expensive gifts should be offered, and these are indeed part of the ritual. The paramount offering is not the gold or the most succulent fruit, or even the head of the finest animal, but the devotee’s own head. It is the loftiest gift, representing the preeminent act of sacrifice. Certainly we understand it figuratively but the real sacrifice of a fine animal, a buffalo, or a goat, is thus often a symbolic substitute for themselves. In all cases the animal sacrifice is a symbol of life, especially human life. And any sacrifice is always about transcendence and is has not merely a cathartic function but it is an attempt to locate human reality in a meaningful cosmos, to affirm life over death, and an attempt to transcend the present.
"Offering to the Goddess Devi" by Christophe Boisvieux
The substances in which the Devi manifests herself through the Durga Puja ritual, such as earth, water, fire, sound, vegetation, and human, these are the very materials with which she is worshipped. All the constituent substances of the cosmos are identified with the Goddess, and these same materials are used to shape her many images. The Great Goddess is therefore not merely the virgin girl in Kumari Pooja , but the pure, potential mother of all life. Thus, She is present in all forms.
Manifest in earth, the Goddess is worshipped with earth through the construction of the earthen altar, her earthen jar body and her clay image.
Manifest in water, she is worshipped with water in her jar body and in her great bath.
Manifest in space and sound, she is worshipped with the sounds of bell-ringing and utterances of adoration. Manifest in vegetation, she is worshipped with flowers and fruit.
Manifest in our life, she is worshipped with the sacrificial offering of life.
It is possible to assume therefore: through the blood sacrifice, the Great Goddess is worshipped with her own blood. The Kalika Purana claims:
"Let the sacrificer worship the victim...When this is done, O my children! The victim is even as myself,... then Brahma and all the other deities assemble in the victim, and he gains the love of Mahadevi,...who is the goddess of the whole universe, the very universe itself."
Devi Durga killing the demon mahisha
It is possible to understand a Durga's victory as an actual event or symbolically. The main thing that Durga was built with the power to destroy all demons. Durga is Power. Pure Power, the Shakti. From this power Devi Durga took form ( She is Adya Shakti and devas did not created her they just summoned her). At the end of battle when the furious bull took a man form, Devi pierced him with trident. Before dying Mahisha worshiped Goddess Durga and whenever we see Devi is being worshiped we also see Mahishasura is standing under her feet pierced by trident at chest.
Being a myth and a part of live religion, this story is of course elusive. There is no "correct answer" to what it is all about, and there are other possible readings. Anyway, we need to continue our inquiry into meaning of our life as spiritual journey, encountering one's personal Minotaur or Mahisha, with hope of Devi's blessing.
Jaya Devi Mahishasura Mardini!
Happy Navaratri to all!