Any animal can be made a pet, so a better question would be: What is the fastest animal? (I'm guessing you mean land animal, so that narrows it down a bit, at least)
Cheetahs can no doubt run faster than any known animal that isn't extinct, but the problem is that they can't sustain that running speed for nearly as long as various other animals.
Peregrine falcons can dive reaching speeds of up to 200 miles per hour (though they are pretty much just going straight down)
As far as land animals that an average person might want as a pet, however, I would suggest a greyhound.
I found a website about this, and it seems the fastest "pet" would have to be a quarter horse, but that's not a house pet. Your next best option would be a Greyhound dog (39.35 mph), then a whippet (35.5mph), then a domesticated rabbit (35.0 mph). Domesticated cats, or your common house cat, was 30 mph.
http://www.infoplease.com/ipa/A0004737.html
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