Wow, turning sen into zen that's reaching far. The two words aren't even related. Sen is sage and Zen is whatever Zen is. Sennin is a loanword from Middle Chinese meaning "Immortal Person", and there's a tale about a Gama Sennin (Toad Immortal) who learned immortality from a toad (yeah I got that from wikipedia: http://en.wikipedia.org/wiki/Sennin). Here's more for your answer:
Originally Posted by senju roshith
Kishimoto: When you talk about ninjas, you invariably talk about frogs. There is a ninja manga whose main character is "Hattori-kun," who (as a joke in the story) does not like frogs, so this connection between ninjas are frogs is a close one [in Japanese Folklore, frogs are considered magical animals, and the oil from a frog's skin is a traditional ointment for wounds and other ailments]. The triad of snake, snail and frog is from Japanese mythology. The snake is more powerful than the frog, the snail is more powerful than the snake, and the frog is more powerful than the snail, making a "paper, rock, scissors" sort of situation. These three are collectively called the Sansukumi.