Having four legs doesn't follow from something being an animal just because there are examples of animals with four legs. A chicken is also an animal.
Having four legs is accidental to being an animal, just as there being an infinite number of elements between any two elements in a set is accidental to its cardinality being infinite.
Having four legs is accidental to being an animal, just as there being an infinite number of elements between any two elements in a set is accidental to its cardinality being infinite.