Korean has a surprising number of sayings built around the frog, and the reason starts with how familiar frogs were. Rice paddies and ponds are where the common Korean frog lives, so for most of Korean history people grew up hearing frogs at night and seeing them in the fields.
So a frog in Korean culture is less one fixed symbol and more a recurring character that Korean speakers borrow to point out human flaws. The three sayings below are the ones you'll hear most.
우물 안 개구리
A frog that has lived its whole life at the bottom of a well assumes the small circle of sky above it is all there is. 우물 안 개구리 is what you call someone whose view is narrow because they haven't seen much beyond their own little corner. The image comes from an old Chinese story, which is why there's also a four-character version, 정저지와 (井底之蛙), that you'll see in writing and in the news. In daily speech, the 우물 안 개구리 version is the one people use.
People often say it about themselves when they realize how little they knew.
"여행을 가 보니까 저는 우물 안 개구리였어요." — After I traveled, I realized I was a frog in a well.
"우물 안 개구리가 되기 싫어요. 많이 경험하고 싶어요." — I don't want to be a frog in a well. I want to try many things.
개구리 올챙이 적 생각 못 한다
올챙이 is a tadpole, and 적 means "the time when." The full saying is that a frog doesn't remember when it was a tadpole. It describes someone who has risen in status, or simply gotten comfortable, and now forgets how hard things used to be for them. You'll hear it a lot about bosses who forget what it was like to be a junior employee, or about parents who forgot what it was like to be a teenager.
"팀장님은 개구리 올챙이 적 생각 못 한다니까요." — The team leader has totally forgotten what it was like to be a tadpole.
"자기도 신입 때 실수 많이 했으면서 개구리 올챙이 적 생각을 못 하네." — He made plenty of mistakes as a newbie too, but he's forgotten he was ever a tadpole.
청개구리
청개구리 is the green tree frog, and it's also the word for a kid who does the opposite of whatever they're told. If you tell them to sit, they stand. Tell them to eat their vegetables, and suddenly the vegetables are the enemy.
The name comes from a folktale most Korean kids grow up hearing where a boy never listened to his mother and always did the opposite of what she said. When she was dying, she worried and asked to be buried by the stream, assuming that her son would do the opposite and bury her on the hill instead. This time, though, he regretted his behavior and followed her words exactly. Now he worries that every rainfall will wash her grave away, and in the story he turns into a tree frog that cries whenever it rains.
People tend to use 청개구리 for someone who never does as they're told, yet is hard to stay mad at.
"우리 아들은 청개구리라서 하지 말라고 하면 더 해요." — My son is a 청개구리, so if I tell him not to do something, he does it even more.
"저는 청개구리예요. 공부하라고 하면 하기 싫어요." — I'm a 청개구리. When someone tells me to study, I don't want to.