A white rabbit, a white elephant, a white mouse...you get the picture.
A zebra...would be racist. Any animal that has been used in the past to describe blacks, like monkey...racist.
Bush was called a monkey or a chimp because his facial expressions resembled monkeys and chimps, as well as his antics, where were very chimp like.
Chimps and monkeys are fun, funny to watch.
Obama is not fun, and not a naturally goofy guy.
Now, call him a black panther, and that may or not be racist...depending on your political bent at the time.