The association of the emperor Nero with frogs is not unique to Plutarch (see my earlier post The Emperor Nero: Viper or Frog?).
A thousand years later, the Golden Legend recorded a medieval folktale in which the emperor gave birth to a frog:
Nero was not unpunished for their [St. Peter's and St. Paul's] death and other great sins and tyrannies that he committed, for he slew himself with his own hand, which tyrannies were overlong to tell, but shortly I shall rehearse here some.
He slew his master Seneca because he was afraid of him when he went to school.
Also Nero slew his mother and slit her belly for to see the place where he lay in. The physicians and masters blamed him, and said the son should not slay his mother that had borne him with sorrow and pain.
Then said he, “Make me with child, and after to be delivered, that I may know what pain my mother suffered.” Which by craft they gave to him a young frog to drink, and grew in his belly, and then he said, “But if [unless] ye make me to be delivered I shall slay you all.” And so they gave him such a drink that he had a vomit and cast out the frog, and bare him on hand that because that he abode not his time it was misshapen, which yet he made to be kept.
Then for his pleasure he set Rome afire, which burned seven days and seven sights, and was in a high tower and enjoyed him to see so great a flame of fire, and sang merrily.
He slew the senators of Rome to see what sorrow and lamentation their wives would make.
He wedded a man for his wife. He fished with nets of gold thread, and the garment that he had worn one day he would never wear it ne see it after.
Then the Romans seeing his woodness [madness], assailed him and pursued him unto without the city, and when he saw he might not escape them, he took a stake and sharped it with his teeth, and therewith stuck himself through the body and so slew himself. In another place it is read that he was devoured of wolves. Then the Romans returned and found the frog, and threw it out of the city and there burnt it.