In the long history of the Subcontinent of India, water buffalo flesh had been consumed, dating back all the way to Harappan culture. Water buffalo is native to Asia. In ancient times, there were plenty of wild water buffalos roaming around southern grasslands of modern day Nepal, India & Pakistan due to massive water resources from the Himalayas and monsoon rain. A 5000-year-old carving depicts the sacrifice of the Water Buffalo
1. Plano convex molded tablet showing an individual spearing a water buffalo with one foot pressing the head down and one arm holding the tip of a horn. A gharial is depicted above the sacrifice scene and a figure seated in yogic position, wearing a horned headdress, looks on. The horned headdress has a branch with three prongs or leaves emerging from the center. https://www.harappa.com Asko Parpola writes: "Early Harappan cultures started moving toward the east and south in about 3000 BCE, and later waves of influence in these same directions came from the Indus civilization. That the Harappan water-buffalo cult [Fig. 2, 3] had reached peninsular India by the late Harappan on Chalcolithic times is suggested by the large bronze sculpture of water buffalo [Fig. 4] discovered in 1974 in a hoard at Daimabad, the southernmost Indus site in Maharashtra. Throughout south India, until relatively recently, village goddesses have been worshipped through water-buffalo sacrifices. The goddesses have been associated with a male deity call [sic] the "buffalo king," represented by a wooden post or a pillar made of stone, or by the pipal tree (Biardeau 2000)." Asko Parpola, The Roots of Hinduism, pp. 175-176, p. 178. Since the beginning of Indian civilization, it has been domesticated for work on the farm and dairy products. Typically the water buffalo has been slaughtered and consumed after animals reached the end of their working life. Old animal meat must be slow-cooked in the broth with spices to break down the tough tissues. The young and prime buffalo, which only the rich could afford, needed little cooking to produce a tender, flavorful result. Some served rare meat as Kachi-la. Historically, it was not a daily diet on their table. Our ancient ancestors did not have the luxury of obtaining meat from the butcher shop. They slaughtered the animals on special occasions and shared meat with family and friends. One of those special days was the harvest moon of October. This time of the year is the season of hunting and harvest.
The question is, why do Newar's eat Water Buffalo? The meat of the Water Buffalo has been the most prized, primary protein source of Newari people for over 2000 years. According to Nepalese history, Mahispal (Avir) dynasty, who were the water buffalo herders, had ruled Nepal for a while. There is no historical evidence of their food culture, but people might have been eating meat since then. There is solid evidence that people ate buffalo meat during festivals since the Malla dynasty, around the 10th century. It can be proven based on our Royal tradition of a yearly offering to Goddess Talaju. Talaju Bhawani deity was brought to the Nepa Valley by Harisimha Dev's family from Simraungadh. The Malla Kings came to adopt the tradition and became the guardian of the deity. To connect the dot with the Asko Parpola theory, Harsimaha Dev's ancestors arrived from southern India. They must have brought the tradition of sacrificing water-buffalo and consumption of the flesh from the south.
Newari people have an amazing culture, and their love for festivals and celebrations are evident through their elaborate food. They have built social structure by trial and error for thousands of years. It is imperative to slaughter animals during special occasions and prepare over 30 varieties of dishes from different parts of the animal. As far as I remember, there were concerns relating to animal welfare, and there were rules before slaughtering the animal. Killing any female animal or a sick one is prohibited. The butcher must observe the health of the animals for three days before the slaughter. Usually, they monitor the eyes, nose, urine, and body movements of the animal.
Butchering evolved from a function performed for families into a service conducted for larger populations. In the middle ages, public slaughterhouses were permitted to open, dating back to the Malla dynasty in the 13th century. Animals were butchered in the open air. (Things are changing these days due to mass meat production and demand.) Later, butchers were restricted to certain neighborhoods. This was due to a large amount of animal waste and blood being dumped into the middle of the street that served as an open sewer. This pattern really disgusted many people. Some stopped eating meat due to the unhygienic process. Some people in Nepal were against animal sacrifices, and Newar people were criticized for consuming Buffalo meat. One might not like everything about others' cultures, but every culture has its own history and culinary traditions, and we can respect those traditions. Research on water buffalo was done at the University of Florida. Here is an article by Boyce Rensberger published in Nov2, 1986 of The Washington Post.