Here's why:
* Insects: Have six legs.
* Spiders: Have eight legs.
* Crustaceans: Like crabs and lobsters, can have a variety of leg numbers, but not 10.
* Centipedes: Have a large number of legs, but always an odd number.
* Millipedes: Also have many legs, but always an even number.
If you're thinking of an animal with ten appendages, it might be an insect like a praying mantis, which has six legs and two long, grasping forelegs.