According to archaeological findings, the lions were quite common in Europe until Pleistocene, but even around 1st century BCE they were still commonplace in Greece, Asia Minor, Caucasus, Middle East and Northern Africa (its Mediterranean coast), so they were well known to Greek and Romans and thus were commonly depicted in the ancient art. Of course, there were older depiction of that animal, many of which are preserved to this day. There are multiple depictions dating back to 23rd century BCE in Egypt, with one of the popular deities, Sekhmet, being portrayed as a humanoid with a lion's head and the other, Bast, usually portrayed as a lioness and later gradually changed into the likeness of a domestic cat. Lion's Gate in Mycenae, erected in 14th century BCE also displays lions as its centerpiece. According to numerous bas-reliefs, lion hunts were also a common pastime of the Mesopotamian royalty, as attested by many depictions from 8th to 6th century BCE. One of the most popular reference to this animal is, of course, one the Twelve Labours of Herakles, namely the defeat of the Nemean Lion the pelt or body of which was a common attribute of that particular hero. Myth of Herakles enjoyed vast popularity thus facilitating various depictions - from sculptures, to mosaic to paintings, found anywhere between Phoenician homeland and Iberia.

Many of these artifacts survived until Middle Ages, so the local artists had quite a substantial reference for depicting lions by themselves even though it can be safely assumed that most of them had little to no chance of seeing a living one in their entire life. The Medici lions, mentioned above is in fact a Roman sculpture from around 2nd century CE that was found in late 16th century and served as a model for its twin, now standing alongside the original in Loggia della Signoria in Florence (earlier in Villa de Rome). This lion and so-called 'Albani's lion', a 1st century CE Roman marble sculpture that joined cardinal's Albani collection in 18th century, are most likely the models for numerous depictions of a lion keeping a paw on a stone ball made since 16th century.

The demand for leonine likenesses was indeed high, given the popularity of the animal in the most common Christian religious text i.e. the Bible, from Samson's fight with a lion and Daniel in the lion's den to the reference to the Lion of Judah to the symbol of Mark the Evangelist. Greek and Roman influences were usually cited as the reason of popularity of the lion as a heraldic charge, with the 'Physiologus' written by an unknown author being often cited as the main source of the animal symbolism, from where lion's association with courage, power and splendour (hence 'king of the beast) come. Of course, the aforementioned association with royalty, hailing from Mesopotamia and also present in Persia and Judea also influenced the association. It is also possible that the lions were already an identification symbol among the people who came to Europe from Pontic steppes and later merged their traditions with similar customs of Gauls and later with the Greco-Roman influences, giving birth to European heraldry as we know it. In other words, due to the ancient and early medieval influences of Greeks, Romans, Egyptians, Hebrews, Scythians and other peoples, Europeans had a good grasp on the existence and general image of lions. Sure, some of the depictions (like the one from Echternach Gospels, written and illustrated most likely in the Lindisfarne Abbey in the late 7th century) were not exactly close to the mark, but they conveyed the general traits of the animal.

EDIT: Resurrected few sentences that were grammatically butchered. The information remained unchanged.