Eustace (ca. 2nd c.), early Roman martyr. According to legend, the Roman general Placidus was out hunting a stag when an image of Jesus on the cross appeared between the animal's antlers, inspiring him to convert to Christianity and adopt the name Eustace. He was tested for his faith by a series of misfortunes, and was later burned alive inside a bronze bull. Though his historical existence is doubtful, he was a popular saint throughout the Middle Ages.
Excerpt from The Golden Legend
So on a day, as he was on hunting, he found an herd of harts, among whom he saw one more fair and greater than the other, which departed from the company and sprang into the thickest of the forest. And the other knights ran after the other harts, but Placidus siewed him with all his might, and enforced to take him. And when the hart saw that he followed with all his power, at the last he went up on a high rock, and Placidus approaching nigh ... And as he beheld and considered the hart diligently, he saw between his horns the form of the holy cross shining more clear than the sun, and the image of Christ, which by the mouth of the hart, like as sometime Balaam by the ass, sp[o]ke to him, saying: Placidus, wherefore followest me hither? I am appeared to thee in this beast for the grace of thee. I am Jesus Christ, [and] I come hither so that by this hart that thou huntest I may hunt thee. ... I am Jesus Christ that formed heaven and earth, which made the light to increase, and divided it from darkness, and established time, days, and hours. Which formed men of the slime of the earth, which appeared on earth in flesh for the health of the lineage human, which was crucified, dead, buried, and arose the third day.
And when Placidus heard this, he fell down again to the earth, and said: I believe, Lord, that thou art he that made all things, and convertest them that err. And our Lord said to him: If thou believest, go to the bishop of the city and do thee be baptized. ... And when he was come home to his house, and had told this thing to his wife in their bed, she cried: My Lord! and said: And I saw him this night that is passed, and he said to me: Tomor[row] thou, thy husband, and thy sons, shall come to me. And now I know that it was Christ. Then they went to the bishop of Rome at midnight, which baptized them with great joy, and named Placidus, Eustace, and his wife, Theospis.