Heracles is said to have had intercourse with the 50 daughters of Thespius or Thestius in one night, all of whom bore him sons (Diodorus Siculus, Library 4.29.3) In another version, one daughter refused, making the number 49 (Pausanius, Description of Greece 9.27.6).
However, in some less impressive versions, this took place over a week (Herodorus apud ...
Several real bird species are called "firebirds":
subspecies of northern oriole, like Baltimore oriole
various birds, having bright scarlet or orange plumage like Scarlet tanager