Once you reach the "Farmlands" district, the Mountain Ninjas will offer their help (actually because a Wee Ninja says We Wee and he likes it). Mountain Ninjas have normal speed but are much stronger and deal splash damage. However, the downside is that a ninja hut will only spawn one Mountain Ninja instead of two. The cost is 80 Ninja Star Cookies.
Feroshi the Dino Slug is available on the district "The Ovens" for certain levels. He will be moving around the road and, for a certain amount of Ninja Star Cookies, he will perform either weak, normal or strong attacks (fire balls). Choose him and select between the three amounts of ninja Star Cookies (25, 50, or 100 in order of strength). However, if you feed him too much, he will take a quick nap of about fifteen seconds.
Lava Ninjas are available starting from the district "Feroshi Mountains". They are a long range type of ninjas and throw fireballs. When a fireball touches an enemy, they are set on fire for a certain time andare dealt damage over time. They cost 80 Ninja Star Cookies.
The Wee Ninjas are available at the start of the game and may seem "normal", but if you want to succeed easily on later districts, build tons of Wee Ninja huts around the road. Wee Ninjas have a normal speed and decent attacks, but cannot be slowed down, stunned, poisoned, or "zombified". One of the best layouts would be two Wee Ninja huts for one other type close range ninja and one type of long range ninja.