Short answers, pulled from the story.
Euhemerus was a Greek mythographer who lived in the late 4th century BC. Euhemerism is named after him because his enduring influence on later thinkers, including the poet Ennius (born 239 BC) and the historian Antoine Banier (born 1673), established him as the traditional founder of the rationalist school of myth interpretation, even though earlier writers such as Herodotus and Xenophanes had expressed similar views.
Euhemerism holds that mythological accounts originated from real historical events or personages. Historical accounts become myths as they are exaggerated in retelling, accumulating elaborations and alterations over time.
Early Christians used euhemerism to argue that pagan gods were merely deified human rulers, undermining the legitimacy of pagan worship. Clement of Alexandria, Cyprian, Eusebius, and others deployed the argument that gods such as Baʿal and the Olympians had once been mortal kings whose memory had been confused into divine veneration.
Euhemerus argued that Zeus was a mortal king who died on Crete and that a tomb bearing his name could still be found there. Callimachus disputed this, arguing in verse that Zeus was eternal and that the Cretans, who always lie, had fabricated the tomb.
In the Prose Edda, composed around 1220, Snorri Sturluson presented Odin as a historical leader originally from Asia Minor whose ancestry traced back to Priam, king of Troy. Snorri described Odin traveling north to establish the royal families of Denmark, Sweden, and Norway.
Euhemerism presumes that myths originated from real historical events or persons. Euhemerization, a term defined by author Richard Carrier, describes the reverse process: taking a cosmic god and placing him at a definite point in history as an actual person who was later deified, effectively inventing history rather than recovering one.