So I have to invent a new animal and explain its effects on other animals the environment and I was wondering how I could start writing that ... just a basic outline ...

I had to do a project similar in Science. So I would start off with "The animal that I have created is called a ___. It has multiple effects on the animals in it's habitat, and the environment around it." And then somewhat list (don't make a list but one by sentence the reasons instead) the problems it can cause / the effects it can have on the territory around this creature.