To my understanding and knowledge plants and animals fall under different elements. Now you could put a blanket over all plants and animals being earth elements, however many do not do this. They break them up to be put as representations of all the different elements.
Water can be represented by the plants that grow in water or near water. Like seaweed or a weeping willow tree. Animals that would represent water would be fish and amphibians like frogs. Birds can be a bit tricky due to wanting to place them in air but water foul would also be a good representation. Mammals like dolphins and whales would also fall under the element water.
Air would be connected to plants that have an air theme like a butterfly bush or any tree for that matter since plant life cleans the air we breath. Animals would be birds, flying insects, bats, and gliders.
Earth is very easy. But things like earthworms, moles, turtles, and anything else that spends it time on or in the ground. Plants, well almost all need earth to grow.
Fire can be harder. Plants would be ones that are connected to the sun, like a sun flower or a burning bush. (I think you are getting the idea) Animals would be fire flies, snakes, or other desert creatures.
The whole idea is that if you can connect the plant or animal to the element then it fits. However make sure you can back up you ideas for if you can not explain them then how can anyone else see what you are thinking. :)