They had no way of knowing which fruits were good to eat, and which were poisonous. Animals are born with this sort of knowledge. Each kind of animal finds and eats the food that suits it. Each type of animal is able to protect itself in its own way. Lions eat meat, not grass, and they use their teeth and claws to protect themselves.
Deer eat grass.
And they can run very quickly to avoid the creatures that hunt them. Each animal has its own way of life, and each knows automatically what to do. Each was especially adapted to live in a certain type of environment. The lion needed grassy plains - it could never survive in the ice and snow of the frigid north. The deer were at home on the forests.