The Martial Arts, and this includes Shaolin kung fu, are passed on through unique strategies of motion. They are, at their source, a mythology created to represent a unique methodology. They are the science of an ancient culture, passed down to us by dance.
Thus, to understand the martial arts, we must ask ourselves what martial science were the ancients trying to pass along. Science is science, however, and I think you will find that the real reasons for the five animals are quite logical. Also, you will find that there is a distinct combat strategy behind the animals.
The tiger is an aggressive animal, and he is designed to go forward. Claws and teeth, he drags down the runner and rends him fiercely. Thus, the direction of the Tiger is forward, and he charges and charges and charges.
The Crane is a lighter animal, giving way as he manipulates and uses legs. He does not run, but rather and deflects the attack to set up his kicks. He does not charge, but fights on the edge of a circle, never letting that circle collapse.
The Snake coils and twists and allows the attacker to get close. The artful deflections of the crane become full blown traps and locks under the deceptive practices of the snake. Thus, the snake gives way without giving way, allowing the enemy to fall into trickiness and deception.
There is some argument whether the fifth animal is a leopard or monkey, and I opt for the monkey. Whichever animal you choose as the best representation of Shaolin Kung Fu, the attribute of the animal is tricky footwork and the ability to acrobat and gyrate out of the fight. Thus, the Monkey (leopard) has the ability to extract himself from the fight and reset the whole thing to whatever animal characteristic he is strongest at.
The fifth animal is the dragon, and this is an imaginary animal which represents the best of the previous four animals. The dragon is the embodiment of understanding and employing all animals so that they work together. Run, manipulate, trick or flee, the dragon does them all, smoothly and at the right time.
The five animals of Shaolin Kung Fu represent motions in space, and a distinct distance of combat. Practice each animal until you have mastered the talent, then blend them all together. This is the fighting strategy of the Five Animals of Shaolin Kung Fu.