You are writing a critical essay, one in which you are presenting an argument - that animals have rights which are being violated - and presenting evidence to support your argument. Your topic sentence is your opportunity to present an overview of your position and why you feel it is important.
Your topic sentence should state that you feel animals have certain rights, some of which are being violated by some people or industries. Continue your introductory paragraph by presenting the illustrations you are going to use to support your argument and briefly explain how these groups violate animal rights.
In the body of your essay, go into more detail. Present examples of ways in which animals are used to test products or procedures that ignores the rights of those animals. If you have figures to show how many animals are killed for the sake of medical research, for example, this would be the place to document that type of animal abuse. Explain what happens during a bullfight and how this ignores the rights of the bull.