Androcles, a Roman slave from the 2nd century AD, escaped from his proconsul master in North Africa due to daily flogging. He fled into a cave to hide, only to discover it was the den of an enormous lion, which was bleeding from a paw.
Summoning his courage, Androcles helped by removing a large splinter and cleaning the wound. The lion then peacefully fell asleep. They shared the cave and food for three years. Later, Androcles was captured and sentenced to be killed by the beasts in the Circus Maximus, but the lion, now his friend, spared him and greeted him happily, like a loyal dog. Recognising the lion, Androcles, terrified at first, was comforted and stroked it. He was eventually pardoned and reunited with the lion.
The story is reported in Aulus Gellius's Attic Nights, 10.6:

<<In the Circus Maximus, there were many wild beasts, and all possessed a prowess or ferocity never seen before. But more than anything else, the immense size of the lions was a source of admiration, and one in particular was more admired than all the others. That one lion, by its impetuosity and the size of its body, by its terrifying and mighty roar, by its muscles and by its waving mane, had attracted everyone's attention and gaze. Among many others, the docile servant of a man of consular rank had been brought in, condemned to be devoured by wild beasts; that servant was named Androcles. The lion, when it saw him from afar, suddenly stopped, almost amazed, and then slowly and calmly approached the man as if trying to recognise him. Then it began to wag its tail calmly and docilely, like a dog celebrating, to rub itself against the man's body and to gently lick his legs and hands with its tongue, the man already half dead from fear. The man, Androcles, amidst the outpourings of that terrifying beast, regained his lost courage and gradually turned his eyes to observe the lion. Then, as if they had recognised each other, everyone saw the man and the lion happily exchanging outpourings of affection. From the imperial box, they signalled for the condemned man to be taken immediately to the emperor, who wanted to know directly from him why the lion had spared him. The guards in the arena, the Praetorians, seized him and took him to Caesar. The emperor, hearing his story, granted him pardon and gave him the lion, and passed around a tablet with the condemned man's statement:

"While my master was governing the province of Africa as proconsul, I was forced to flee from his unjust daily floggings, and I retreated to the deserted countryside and sandy plains. Then, since the midday sun was fierce and burning, I came across a remote and hidden cave, and I entered it to hide. And not long after, this lion arrived at the same cave, with a wounded and bleeding paw, uttering groans and groans that expressed the torment of the wound. After the lion, having entered what was his den, saw me trying to hide in the back, he approached me meekly and tamely and seemed to show me and offer me his raised paw as if to ask for help. Then I extracted a huge splinter of wood embedded in the sole of his paw, I drained the infected blood from the bottom of the wound, and with some care, now without great fear, I dried it thoroughly and cleaned it of the congealed blood. Then the lion, comforted by my work of healing, placed his paw in my hands, lay down and fell asleep, and from that day on, the lion and I lived for three whole years in the same cave and even on the same food.">.