I thought I saw Lotad having six legs, and it's not even based on an insect, so why??
I guess its to make the design more interesting... After all, Lotad is practically a lilypad with a smiley face. Quite a few Pokemon have extra things, such as Giratina, who has 6 legs as well.Vaporeon has 3 ears (or you can call them fins), and Ninetails has (clue in the name :P) nine tails!
In Lotad's PokeDex entries they say that Lotad can ferry small Pokemon across water. Well I guess that the reason why Lotad has 6 legs it's because it needs the strength from them to carry the Pokemon across water.
Hope I helped!
Yes he does its to help him float better and swim faster away from predators.