Short answers, pulled from the story.
Set is an ancient Egyptian deity of deserts, storms, disorder, violence, and foreigners. He played a dual role: as a protector of Ra against the chaos serpent Apep, and as the murderer of his brother Osiris in the Osiris myth. His name in ancient Greek is Sēth.
The Set animal has not been definitively identified as any known species. It features a downward-curving snout, long squared-off ears, a thin forked tail with inverted-arrow fur tufts, and a slender canine body. Proposed identities include the aardvark, African wild dog, fennec fox, and giraffe, among others.
The conflict between Horus and Set concerns the kingship of Egypt. Depending on the tradition, Horus is either Set's nephew seeking revenge for the murder of Osiris, or Set's brother. The dispute was adjudicated before the divine council known as the Ennead and, in the Contendings of Horus and Set, dragged on for eighty years.
The Hyksos, a Near Eastern people who ruled Lower Egypt during the Second Intermediate Period (1650-1550 BCE), adopted Set as their patron because they found him most similar to their own chief deity, Hadad. Their king Apophis worshipped Set exclusively, building a temple to him at Avaris and making daily sacrifices.
According to Egyptologist Herman te Velde, Set was demonized after Egypt's conquest by foreign powers during the Third Intermediate and Late Periods. Because Set was traditionally the god of foreigners, he became associated with Egypt's foreign oppressors, including the Kushite and Persian empires, and his negative attributes were increasingly emphasized.
The founder of the Nineteenth Dynasty, Ramesses I, came from a military family in Avaris with strong ties to Set's priesthood. Several Ramesside kings bore names honoring the god, including Seti I, meaning man of Set, and Setnakht, meaning Set is strong. Ramesses II erected the Year 400 Stela at Pi-Ramesses to commemorate the 400th anniversary of the Set cult in the Nile delta.