Whenever I see this picture I think of this:
| Creature | Swims | Walks | Flies |
| -------- | ----- | ----- | ----- |
| Dog | X | X | |
| Fish | X | | |
| Bird | | X | X |
| Duck | X | X | X |
Notice how for each combination of swimming, walking and flying, there is a living creature that can do just that, with the exception of swimming and flying.
Sure there are flying fish, but they glide rather than fly. Did evolution really forget to cover this niche?
huh, is it restrictive to say that you need some number of legs to be able to walk?
In this context, one way of looking at it is just propulsion against the ground vs. through the air or through the water. By that definition snakes, or even something like snails, are walkers.
And, for what it’s worth, even if you require legs there are edge cases. Is inchworming a kind of slither, or a kind of walk?