Answer:
Actually the only animals that would be affected by a magnet (unless they swallowed metal or another magnet) would be sharks and rays. Only sharks and rays have a sixth sense that can detect minute amounts of electricity. This sense is helpful because every time a muscle moves, it gives of a tiny amount of electricity that can be detected by the sense. this sense can only pick up the pulses from a few feet away. Magnets interfere with this sense and leave them disoriented and then they try to avoid it. This sense is possible because of little pores in the skin called the ampullae of lorenzini that have a jelly like substance inside them and the jelly picks up the electricity. Unfortunately that is as far as I know about this sense.
if you do not believe me look up the ampullae of lorenzini for yourself ** **** It is thought that homing pigeons use magnetic fields, when making long distance flights, so losing that ability or having it warped with strong artificial magnetism, before the bird has established it's whereabouts, could mean it never finds it's way home.