I will try to answer this question by looking at how the lion became the king of animals in Middle Age Western Europe.
Sources : PASTOUREAU (Michel), “Le sacre du lion”, in Une histoire symbolique du Moyen ge occidental, Paris, 2014, p.54-72.
PASTOUREAU (Michel), L'Ours : histoire d'un roi déchu, Paris, 2007.
Presence of the lion in Western Europe in the Middle Ages
Firstly, it is necessary to know what Medievals knew about lions. The last wild lions in Western Europe, probably living in Spain, became extinct thousands of years before our time. The Romans themselves brought their lions from North Africa or Asia Minor.
This does not mean that Europeans never saw them. Thus, itinerant animal trainers could have lions. There were also menageries belonging to kings, great lords or abbeys, and then increasingly to chapters, cities or prelates. The aim was to show their power. The number of lions in menageries increased during the Middle Ages, and they were soon joined by leopards and panthers.
The lion, a very popular figure
The lion was an extremely popular figure in the Middle Ages. It can be found in many iconographic representations, in first names (Leo, Leonardus, Leopoldus), family names (Leonelli, Lionnard), and especially in coats of arms (15% of coats of arms have a lion!, the second most represented figure is found in 6% of coats of arms and the second animal, the eagle, in only 3%). There is even an expression "He who has no arms, bears a lion". All the rulers of Christendom (apart from the Emperor and the King of France), adopted, at one time or another, a lion in their arms).
In short, lions were very popular. But if you look closely, you can see that there was a real explosion in the late 11th and 12th centuries. Before this date, the lion is less present and does not seem to qualify as the king of animals in Western Europe.
The lion, king of animals?
The title of king of animals for the lion is actually quite recent and the result of a long evolution. We will, I am very sorry to say, have to go back to the legacy of earlier periods.
The biblical tradition
The Bible was written in the Near East, a region where lions, leo persicus, were smaller than their African counterparts. Of course the lion population was declining, under the Romans they were already rarer (and they had almost disappeared by the time of the crusades). But the lion is quite present in the Bible and his image is ambivalent.
There is a positive one: defeating a lion is a great achievement, and the lion is in the emblem of the tribe of Judah.
But there is also a much more negative view: for example, in the book of Psalms: "Save me from the lion's mouth", or in the New Testament: "Be vigilant: your adversary, the Devil, like a roaring lion, prowls about, seeking whom to devour. Resist him, firm in your faith". This image is therefore strongly taken up by the Fathers of the Church who make him a diabolical character. To defeat a lion is to defeat Satan.
We are still a long way from the king of animals.
Greco-Roman and Eastern traditions
The Romans, and especially the Greeks, were very familiar with lions (the last lions in Greece are thought to have disappeared around the fourth century AD). But they did not generally make them the kings of animals.
Except perhaps for some Latin bestiaries which were to be of great importance for 12th century’s authors. Derived from the Greek Physiologus compiled in Alexandria in the 2nd century AD, they are very much influenced by oriental traditions which place the lion as king of animals.
Celtic and Germanic tradition or the danger of the bear
Celts completely ignored the lion, but Germans and Scandinavians gradually integrated it into their bestiary even before their Christianisation. The symbolism of the mane is strong, while long hair is often associated with power.
But in both cases, other animals dominate (raven, boar) and especially the bear. The latter is the royal animal (think of Arthur, which means bear), the one that kings kill to prove their values. It is an ambiguous animal, considered to be strong, violent and the closest to man. It is also extremely present in pagan cults, even during High Middle Ages.
This naturally worried the Church, which did everything it could to combat the image of the bear: It demonised the animal (St Augustine "ursus est diabolus" and Pliny were used to make it a dissolute being who raped women and accompanied the devil, and all vices were attributed to it), domesticated it (St Armand, St Corbinien, St Colomban all domesticated bears) and ridiculed them (around 1000, bear dancing became more and more popular without the Church, which was normally opposed to animal shows, saying anything ; bears were no longer present in royal menageries and were no longer propers kingly gifts as they were under the Carolingians...).
But the Church then looked for a more exotic and less cult-like replacement: the lion.
Crowning of the lion
From the end of the eleventh century onwards, the lion experienced an extraordinary promotion. Certainly, Ambrose, Origen and Raban Maur had already begun to insist on the positive figure of the lion based on the New Testament. Isidore of Seville, in the sixth century, made him a king (but not of animals). But from the 11th century onwards, the lion really became an extremely positive figure, with many qualities attached to it: courage, pride, generosity, justice; plus Christian characteristics: charity, oblation, mercy. Above all, the lion became more and more a Christ-like figure.
Problem: what to do with the negative aspects of the lion found in the Old Testament? Medievals simply attributed them to the leopard (not our leopard, but an imaginary one). In heraldry, the latter is only a lion with a frontal head (the lion is in profile). The leopard is then a bad lion, in the Arthurian novels the good knight has a lion shield, and the bad one a leopard shield. For the anecdote, this explains why the Plantagenet leopard, adopted under Richard the Lionheart in 1194-1195, gradually became a problem for the English kings. During the Hundred Years' War, the French heralds attacked the English leopards. This is why, without changing their coats of arms, the English kings, from Richard II onwards, began to use the term lion.
In any case, the lion, rid of its negative aspects, can finally be crowned. In the 13th century encyclopedias of Thomas of Cantimpré, Bartholomeus Anglicus or Vincent of Beauvais, the lion is rex animalium.
He is now alone at the head of animals on Noah's Ark (the bear, who accompanied him, is now demoted to 2nd or 3rd place), and in the famous novel Reynard the Fox (12th and 13th century versions), Brun, the bear, is a not very intelligent baron who is ridiculed by the fox, while Noble, the lion, is now king.