I recently came across some pages from a German folio that tell an odd story:
Um... okay??? This is odd for even the most scandalous gossip of emperors. I can't find any older source, or, really, any other source that relates this story beyond the one medieval account here.
What's the context of this? Is it connected to/inspired by other stories? Were people making up new stories about the Roman emperors in the 1410s? What kind of book is this?
Welcome to the wacky world of medieval character assassination!
The standard attribution for this story is to Jacobus de Voragine's Legenda aurea or Golden Legend. Written in the mid-13C in Latin, by the end of the Middle Ages its vernacular translations had made it THE most important and widespread collection of medieval hagiography (saints' lives, told to inspire devotion/veneration and in some cases imitation). The Nero and frog story here is found in the Life of St. Peter, as part of a catalogue of reasons that Nero (who has just killed a couple of early Christians) is The Worst.
The gist of Jacobus' version is that Nero thinks he is the master over nature and reason, so he can do the unnatural and irrational thing of giving birth on his own. His doctors use, it is implied, sorcery to have him drink a frog and then the frog grows in his stomach. Nero panics at the thought of labor, threatens to kill the sorcerers-doctors, and they make him throw up the frog (give birth through his mouth, i.e. even more unnatural, and also reflecting other).
The version here is from Jans der Enikel's world chronicle from the late 13th century, with the language updated to reflect the 15th century manuscript creation. Here's the original, starting in l.23101-23130. The cognate of "krot" or "krote" is toad. According to the Getty Museum's website, Jans' Weltchronik was one of the ones brought in to complete the base text here, Rudolf von Ems's unfinished world chronicle. This mishmash of texts and retelling of stories is completely par for the medieval course. It's often helpful to think of medieval manuscripts as Tumblr accounts, "reblogging" stories and excerpts from earlier authors into a "new" text.
Jans' version has a couple differences from the original, like the emphasis on the frog as Nero's son. Ending up in the Tiber is also a change. In medieval legend, throwing someone else into the Tiber is usually a sign of how terrible they are--Pilate ends up there in a few stories--but my guess is that something else is going on here. The frog's place is naturally in wet nature. By running to the Tiber, the frog proves that Nero is not the master over nature and reason.