These idioms are based on compound nouns related to animals.
A person who ...
is ...
is a dark horse
someone who is clever or skilful in a way that no one knew or expected
is a lone wolf
someone who does not mix socially with other people
is a cold fish
someone who is not very friendly and does not show their feelings
is/acts as a guinea pig
someone who acts as a subject in an experiment or trial of something
is a party animal
someone who loves parties and socializing
would love to be a fly on the wall
someone who would love to be present to see an important private or secret event
People's characters and their actions
John's bark is worse than his bite (he may seem fierce/tough, but he is not really). You don't need to be afraid of him. He wouldn't hurt a fly (is totally harmless and would never hurt anyone). Larry's really got the travel bug ever since he won that holiday in the Caribbean (a strong desire to travel). She's working very hard for her exams. She's really got/taken the bit between her teeth (has started to work/act in a very determined way (a bitis a piece of metal put between a horse's teeth to control it). They've given me free rein to do what I like in this new job (complete freedom).
Tim: How did you know I was getting married?
Linda: A little bird told me (said when someone has told you a secret, but you do not want to say who).
As soon as the meeting was over, we all made a beeline for the food. We were so hungry! (went quickly and directly to). I always have butterflies in my stomachjust before an exam (feel very nervous). I made a real pig of myself last night, so I'm not going to eat any lunch today (ate too much). I have so much work to do these days. I'm just chasing my tail trying to catch up (doing a lot, but it is all pointless / without effect).