Even in the ancient sources, the story of their birth has a huge number of versions. One of them, described by Plutarch but attributed to a guy called Promathion really stands out as it really includes a giant penis.
Romulus and Remus are quite well known mythical founders of Rome. They are the twins rescued by the she-wolf who later restored their grandfather to the throne of Alba Longa and went on to found Rome. Romulus, after killing his twin, was proclaimed the first king of the new city. Just as the mainstream story known from Livy and others, this one starts in the city of Alba Longa. The city was close to Rome and according to tradition was founded by Aeneas on his journey from the fallen Troy. The reigning king is here called Tarchetius and, just as his equivalent Amulius from the mainstream version, the man is a cruel tyrant.
One day a strange thing appeared in the fireplace. There was a huge phallus in the hearth. To avoid any confusion, I really am talking about a long penis, with no man attached to it. While miracles are often a good thing, the king was kind of nervous about this one and decided to get rid of the penis in the fireplace. He consulted the local Oracle and got an actionable piece of advice. A virgin was supposed to have sex with the thing and it would go away. As you might have guessed, the object didn’t just appear to have fun with young and horny Alban virgins, but it would leave the girl pregnant and the child would grow up to be a man unmatched in bravery and strength.
As an obviously responsible father, Tarchetius nicely asked his own daughter to have sex with this thing. The daughter had no intention to do so – a penis in the fireplace is hardly the desired husband for a decent ancient princess. She ordered her young female servant to take her place and the servant did just that. The king was outraged. If he orders someone to copulate with a giant penis, he expects it to be done without question. He swiftly sentenced the daughter and the servant to death.
Before they were executed, the king changed his mind. Or better still, he was told to change his mind. Vesta, a goddess, appeared in his dream and made perfectly clear that the girls were not to be killed. Tarchetius knew better than to disobey gods, so he cancelled the execution. He even promised the girls an opportunity to marry, but under one tiny condition – they just had to weave a blanket first. To make it more fun, they were supposed to stay in jail until the blanket was done and to make It more challenging, their work was undone every night. If you ever have a feeling that your parents aren’t supportive enough, just remember this guy.
Nonetheless, the servant girl got pregnant with the penis and gave birth to beautiful twins. They were left to drown, but were saved by a wolf and later deposed Tarchetius, founded a new city and named it Rome. The rest is history.
Plutarch – Parallel Lives: The Life of Romulus