Grasshopper

Aside from the green insect with long legs, a "grasshopper" is also a slang term for a student or someone who is inexperienced.

This usage became popular due to the American martial arts TV series Kung Fu, which debuted in 1972. In the show, the wise master uses it to refer to his young trainee. Even though many people aren't aware of this origin, it is still commonly used today and always in a joking way.

Teddy bear

Bears are usually thought of as big, scary animals. On the other hand, teddy bears are soft, cute toys for children. Calling a person a "teddy bear" is actually a combination of these two ideas. 

Usually, the person being talked about may seem scary or mean to people who don't know them, either because of their size or their manner. However, in reality they are very kind.

Cougar

Cougars, also called mountain lions or pumas, are large cats that live throughout North and South America. They hunt for animals like deer, goats and sheep in addition to smaller animals like rodents.

However, human "cougars" hunt for something completely different! Since the early 2000s, it has become a slang term for women who like to date much younger men. 

Weasel

Weasels are small mammals with long bodies and short legs that can be found in countries all around the world. However, when referring to people, a "weasel" is a person who is sneaky or who cannot be trusted. 

Additionally, there's the expression "weasel one's way out of something," which means to escape responsibility or a difficult situation.

Mole

Moles are small animals that are well known for digging holes and traveling underground. There's a popular arcade game based on them, and they have even inspired a character in the Super Mario games.

Because they are known for moving in places that are difficult to see, we use the term "mole" to refer to a kind of spy. In particular, moles are people who enter a country, organization, etc. but are secretly sharing information they gather with a rival.

Lone wolf

Wolves are known for living and traveling in packs. So calling a person a "lone wolf" is another way of saying that they are often by themselves.

However, the nuance is less about loneliness and more about independence; perhaps the person is just more comfortable alone or does not need or want assistance from others.

Unicorn

Unicorns are legendary creatures that look like horses but with a single horn sticking out of their foreheads.

Because they are from fantasy and don't exist in real life, this term is often used informally to refer to someone or something that is very rare and almost impossible to find.

Rat

Unlike mice, which some people think are cute, rats are usually disliked and thought to be dirty or scary animals. So when referring to people, it's also negative; a "rat" is a person who cannot be trusted or is generally a bad person.

To "rat on" someone (or to "rat them out") means to betray them by sharing sensitive information that was supposed to be kept secret.

Additionally, it can be used to refer to a person who spends a lot of time in a particular place. This usage is not as negative as the ones above.

Shark

Sharks, of course, are large and very dangerous fish who are known to attack humans. Jaws, a film about a killer shark and one of the first “summer blockbuster” movies, made a huge impact when it was released in 1975.

Human “sharks” are business people who will do anything to achieve success, even lie, steal and cheat.

Workhorse

Workhorses are horses used for work instead of racing or riding. They are known for being strong and dependable.

When talking about people, it refers to a person who can do good work even in difficult conditions or for long periods of time. 

This term can also be used for machines.