In Egyptian mythology, the serpentine demon representing chaos and opposing the sun god Ra is Apep, also called Apophis.
Apep is the great serpent of chaos, darkness, and disorder in ancient Egyptian religion. Each night, as Ra’s solar barque traveled through the underworld, Apep was said to attack or obstruct it. The sun’s successful emergence each morning symbolized the restoration of order and the defeat of chaos, so the myth gave cosmic meaning to the daily sunrise.
Egyptian texts and images portray the enemy as an enormous snake or dragon-like serpent. Priests performed rituals intended to assist Ra and repel Apep, including spells and symbolic destruction of wax or clay figures. Apep was not normally worshipped as a god; instead, it represented the forces that threatened maat, the principle of truth, balance, and right order.
The name Apep is the Egyptian form, while Apophis is the Greek rendering. That makes Apophis a valid alternate name, not a different mythological opponent. Set sometimes helps defend Ra against the serpent, which is why he can be a tempting wrong option, but Set is not the canonical answer to this question. Sobek is a crocodile deity, and Anubis is associated with funerary protection and the dead.