Question

how does a cow called ruminent

Answer 1

Ruminants are mammals that are able to acquire nutrients from plant-based food by fermenting it in a specialized stomach prior to digestion, principally through microbial actions. As cow also chew its food for a long time and has four chambers of stomach it is said to be ruminants. 

Answer 2

A cow is called a ruminant because they eat grass and store in a separate portion of their stomach called rumen.Food gets partially digested in rumen..
Thus they are known as ruminants.

Hope it helps.