Proteus was an ancient Egyptian king who is mostly known for his involvement in an alternate version of the story of Helen of Troy.
In book II of The History by Greek historian Herodotus, Proteus is said to be from Memphis, succeeded Pheron to the throne, and was succeeded by Rhampsinitus (Ramesses III, as he was known by the Egyptians). When Paris took Helen from Sparta, wind took him off course to Egypt. While there his slaves deserted him and told the authorities what Paris had done. Word of his crimes reached Proteus, who then took Helen and seized the treasure Paris had stolen from Menelaus, intending to return her and the treasure to Menelaus when he arrived. The Trojans are unable to prove to the Greeks that they do not have Helen, and it is not until Troy is defeated that they are believed. Menelaus then goes to Egypt; he is treated well by the court of Proteus, and is reunited with Helen. However, upon his departure Menelaus sacrifices some native children and becomes hated by the Egyptians.
Herodotus also makes references to Homer's Iliad and Odyssey, claiming Homer must have been aware of this version of events despite using the more common story.
Another take on this story is presented in the tragedy Helen by Euripides. In Euripides' version, Hera had Helen taken to Egypt by Hermes, and she created a phantom replacement of Helen which Paris takes to Troy. The play takes place when Menelaus arrives at Egypt after the war. Here Proteus had safeguarded Helen throughout the Trojan War, but is dead before the play begins. It opens with Helen visiting his tomb. According to Euripides, Proteus was married to the Nereid Psamathe, had a son Theoclymenos, and a daughter Theonoe who was a gifted seer. Theoclymenos became the new king of Egypt after Proteus and had intentions of marrying Helen.