Heterochromia

The term means variation in coloration, and is perhaps most striking when it involves coloration of the iris.  Wikipedia has the details re the mechanism and corollary conditions, and Environmental Graffiti has collected an assemblage of photos of cats and dogs.

People can also have heterochromia - Dan Aykroyd, David Bowie, Kiefer Sutherland and Christopher Walken are listed as examples.

Cat photo credit Keith Kissel, dog photo  credit Jeffrey Beall.