The expression, to lead a dog's life, describes a person who has an unhappy existence.

VOA: special.2010.05.30

The life of a dog or a cat or a chicken consists of appetites, cravings, desires, and their gratifications.

狗，猫，小鸡的生命,都充斥着各类嗜好，渴望，欲望，以及满足感。

耶鲁公开课 - 公正课程节选

You would say we'll take the dog to the vet, and end the dog's life.

你会把狗送到兽医那里，然后结束它的生命。

普林斯顿公开课 - 人性课程节选