Most caterpillars are mildly poisonous since they only eat a single type of plant so they are immune to the plants poisonous effect. That gets into their fleshy hotdog body. Unfortunately most birds are also mostly immune.

there are definitely some ninja-inspired caterpillars out there

Due to Newtons 46th law of awesomeness, Ninjas are still cooler than spikes, but still are pretty dang cool.