Hidden Shifters by David Dann

Invading aliens on Planet A7 have divided native hybrid creatures into two categories: harmless and dangerous. Shapeshifters, capable of assuming any form, are considered the largest threat. Hidden in sheep form, shapeshifter Adrian Walker works as a janitor at the hybrid prison. When he is promoted to warden, he has no choice but to comply. After his shapeshifter girlfriend is taken prisoner, he learns she is part of an underground movement to overthrow the aliens. Determined to rescue her, Adrian agrees to become the inside man for the resistance. As their plan unfolds, he must decide if he will continue to play it safe or risk it all—not only to save his girlfriend, but also the future of the planet.