Animal speed is difficult to measure, and superlatives such as "slowest" are hard to determine. For example, one could argue that corral, which is considered an animal, is the slowest mover because it does not move at all. For those who think that an animal must move to be considered slow, the notoriously slow garden snail may be the slowest, moving at speeds of .03 miles per hour.
The considerably larger three-toed sloth also moves quite slowly, typically maxing out at speeds of well under a quarter a mile per hour. The giant tortoise is also incapable of moving more than about .20 miles per hour.