this is very simple.

animals (unless corrupted by man) are not capable of sin, because they are one with the garden.

adam and eve ate from the tree, and so, fell from eden into this world of sin, and took humanity with them.

sins are an illusion, animals dont sin because they are totally insynched with thier dna. humanity however are not, we are out of synch with our dna, this is why we feel as we dont belong, or what are we, where we came from etc etc.

when mashiach comes he will fix all of this.