Anthropomorphic

"Anthropomorphism, or personification, is attribution of human form or other characteristics to anything other than a human being." (From wikipedia ) In the case of mpreg, it is often projecting human characteristics on an animal, such as human speech, wearing clothing, being fully human aside from notable animal features, being bipedal, etc.. Fans of this are often known as "Furry" or "Furries".

Biological Explanations
Many of the biological explanations for human mpreg hold for anthro mpreg.