새끼 can be very rude and offensive if used improperly. All duolingo users should be cautious with this word if they are not familiar with it
I recognised this word from dramas when the subtitles say "You son of a..." XD
The term "colt" is only used to describe the male baby horse. "Foal" is the proper translation of this phrase to its English equivalent.
Just asked my husband and he said people do use this term (he was born and raised in Korea).
i feel guilty almost, formally learning this word since i've only heard it used as a swear word before lol