Police in India's northern Uttar Pradesh state have arrested three people allegedly involved in the killing of the endangered Ganges river dolphin after a video of the grim incident went viral on social media, according to local police.

Investigating officer Akhilesh Pratap Singh told CNN that the dolphin's carcass was found on the embankment of the Sharda canal in the Pratapgarh district on December 31.

On Thursday, three individuals allegedly seen on the video were arrested, police said. Nine people also seen on the video are currently still on the run, according to Singh.

The video shows a group of men with sticks and an ax bludgeoning the dolphin to death. Some laughed as the dolphin's near lifeless body was pulled around by its fin in the shallow water.

Forest officer Varun Singh, who was part of the initial investigation, told CNN the dolphin had been an adult Ganges river dolphin.

The Ganges river dolphin was "once present in tens of thousands of numbers" but has now "dwindled abysmally" to a population to less than 1,800 in the last century, according to the World Wildlife Fund (WWF) India.

"Living in one of the most densely populated parts of the world, the Ganges river dolphin faces threats from agriculture and industrial pollution and other human activity such as dam creation, irrigation projects and fishing," according to the WWF, who says that development "divides and isolates populations.

"The rare freshwater dolphin is essentially blind, and is known locally as "susu," a reference to the noise it makes when it breathes, according to the WWF.

When it hunts, the dolphin emits ultrasonic sounds that bounce off fish and other prey that allows them to visualize an image in their mind.

Female Ganges river dolphins give birth once every two to three years to only one calf, according to the WWF.