The cause of this continual comparison of human beings with animals is that long ago, when these expressions first began to be used, animals, and especially wild animals, played a great part in the lives of the people. In the Middle Ages great parts of England, now dotted over with big towns, were covered with forest land. Wolves roamed in the woods, and the fighting of some wild animals and the taming of others formed a most important part of people's lives. The same thing was, of course, the case in other countries. So familiar were people in those days with animals that they thought of them almost as human beings and believed that they had their own languages. It was people who believed these things who made up many of the old fairy tales about animals--stories like "Red Riding Hood" and the "Three Bears."
We often say that we are "as hungry as a wolf;" but we who have never seen wolves except behind the bars of their cages at the Zoological Gardens do not know how hungry a wild wolf can be. Those, however, who first used this expression thought of the lean and hungry wolves who prowled round the farms and cottages in the hard winter weather, driven by starvation to men's very doors. We also have the expression, "a wolf in sheep's clothing." By this we mean a person who is really dangerous and harmful, but who puts on a harmless and gentle manner to deceive his victim.
Another use of the word _wolf_ is as a verb, meaning to eat in a very quick and greedy manner, as we might imagine a hungry wolf would do, and as our forefathers knew by experience that they did do. Most of the people who use the names of the wolf and the fox in these ways do not know anything of the habits of these animals, but the expressions have become part of the common language.
The same thing is, of course, true about the lion, with which even our far-off English ancestors had never to fight. But the lion is such a fierce and magnificent animal that it naturally appeals to our imagination, and we find numerous comparisons with it, chiefly in poetical language. We say a soldier is as "brave as a lion," or describe him as a "lion in the fight."
A less complimentary comparison is an expression we often hear, "as stubborn as a mule." Only a few of the people who use this expression can have had any experience of the stubbornness of mules. Sometimes a stubborn person is described quite simply as a "mule." Another compliment of the same sort is to call a person who seems to us to be acting stupidly a "donkey."