You can refer to life forms as a "living thing" but not as a "thing." Things have a solid form. As such, gases such as air or liquids such as water aren't things.The word things is very commonly used to describe non-specific intangible entities such as the atmosphere in a room. For example, "things were getting weird."Things is often used as a catch-all word. For example, if you listed "things that are fast" you might include people and animals in the list. This is not suggesting that people are things but is just informal language whereby things is used as synonym of "entity."
People, Places and ThingsIt is common for dated English materials to ask students to sort nouns, including animals into the categories "people, places and things" whereby animals are "things." This relates to 19th century English whereby it was previously more common to refer to animals such as a dog as a thing. This is increasingly rare in modern English such that this categorization has been updated to "people, places, things and animals."
CounterexamplesThe following are specifically not things.
Intangible entities such as an emotion
Large entities such as a planet
Tangible, solid, non-living entities that aren't too big.
An inanimate physical object as distinct from a living being.
Non-specific intangible entities such as the atmosphere in a room.
Confusingly, the word thing is commonly used as informal synonym of "entity." In this context, anything can be a thing.