This depends on what animal you are talking about. The fastest LAND animal is the cheetah, for it reaches speeds of 65 to 70 miles per hour. The fastest SEA animal is the sailfish, reaching speeds of 75 miles per hour. The fastest TREE animal is the squirrel at 12 miles per hour. The fastest AIR animal is the peregrine falcon at 200 miles per hour, which you could consider the fastest animal based on the condition of your question.
If you talk of the fastest animal of all the world (diving or moving forward) it's the peregrine falcon with his 250 kilometers per hour.