The Highwayman poem is purely fictional. The author used imagination and literary devices to craft the tale. It doesn't have roots in real events or people.
The consensus is that 'The Highwayman' is not a true story. It's more of a literary composition crafted to entertain and engage readers through fictional characters and events.
One famous - ish story could be the Legend of Green Jack. He was a highwayman who was said to have a green tinge to his skin, as if he had some sort of otherworldly connection. People believed he was cursed and had to rob to survive. He would target wealthy merchants on the highway. His greenish appearance made him seem like a part of the forest, and he was very difficult to catch as he could blend in well with the trees and bushes at night.
Typical key elements include the dashing highwayman figure, often with a sense of mystery and adventure. There's usually a romantic plot, perhaps a love story between the highwayman and a female character. And the setting, like the old roads and countryside where the highwayman operates.