The Lightsaber Bloon is a Bloon, that when popped, extends into a Lightsaber (horizontally). This lightsaber will go through blocks (including metal ones) as well as Bloons, but it will not destroy the walls. The Boomerang Thrower's upgrade Lightsabre Thrower in Bloons Tower Defense 4 is based on this Bloon.
It does not appear in Bloons 2, as well as the Lightsabre Thrower being replaced by Glaive Lord in Bloons TD 5. This is most likely due to copyright problems, in particular as a trademark issue with Star Wars.