In greek mythology, who was punished by poseidon for boasting about her beauty?

Cassiopeia

Explanation:

Cassiopeia once recklessly offended the beautiful sea goddess, daughter and granddaughter of the mighty god of the sea of Poseidon. Cassiopeia stated that she was more beautiful than all the sea nymphs together. For revenge, Poseidon sent a dreadful sea monster , destroying her husbands subjects.

King Cepheus was concerned, so he asked for the help of  oracle. The only way out was to sacrifice his daughter Andromeda to the sea monster. Thus, with the pressure of the people, Cepheus left his own daughter at the mercy of the sea monster. However, the goddess of Destiny did not like it, so at the moment when the monster was approaching Andromeda she placed Perseus there. A brave hero who was just returning from Gorgon Island where he killed the monster Medusa. Perseus stated without hesitation that he would save Andromeda if he subsequently got her for his wife. Andromeda's parents were overjoyed, and Perseus prepared to fight the sea monster. Perseus put on his feet the winged sandals that allowed him to fly, and armed himself with a magic curved sword that enabled him to win every battle. Yet with all these magical weapons, Perseus had much trouble mastering the powerful sea monster

Cassiopeia

Explanation:

According to a Greek legend, the sea god Poseidon placed the figure of Cassiopeia among the stars. It is said that Cassiopeia has a ridiculous upside-down position to punish her for having been pretentious. Cassiopeia was very proud of her beauty.