I encountered this on a beach in Chandipur, along the Bay of Bengal in India. Quite a few of them were scattered in different locations. Identification of this controversial object is appreciated.
There are over 1700 known species, all inhabiting the sea-bed, mostly in deep waters, but can be found on the continental shelf in many areas.
An echinoderm (alongside sea-urchins and starfish), belonging to the family Holothuroidea, they distinguish themselves from sea urchins by "lying on their sides". They're radially symmetrical with a weak lateral symmetry - they have a preferred "top side", having leathery skin and a lack of external appendages (legs, eyes etc.).
They come in quite a variety of colours, most with tube-feet on the surface. They have a mouth-end where can sometimes be observed everting their delicate tentacle-like mouthparts to feed. They are also fed upon as larvae by small fish, and as adults by humans in many parts of the world where they are dried and sold in markets.
The specimen in the photo is a smooth variety, and appears to have line or plastic thread wrapped around it's hind parts - perhaps from local fishing.