My black lab is more of a person than any zygote, I assure you. But I understand the question. Why do some liberals get wild about animal rights but at the same time are pro-choice?
Okay so here's my attempt to answer your question (keep in mind that I am not a crazy animal rights person. As a liberal living in Wisconsin I fish/hunt/kill ants/etc): There are plenty of laws to prevent humans from being abused or killed. In comparison there are very few laws that protect animals. So for people who love specific (or all) animals they want laws to protect those animals as well. Only the very extreme animal rights activists believe animals have more value than humans.
But that's just their view on animals. It's completely possible to want to protect animals AND be pro woman's choice. It's not about comparing the life of and animal to that of an unborn child.