Neurons are featured in Yoshi's Story. They appear to be large multi-colored worm-like creatures with whiskers on the top of their heads. There are 4 types:
Red with black spots (goes down when you ground pound on it)
Light blue with blue spots (curves after you jump on it)
Yellow with magenta spots (goes straight up and straight down)
Pink with purple spots (stays in the air but tilts side to side sometimes)
All of them have eyes on the top of their heads covered by a light blue eyelid. When you touch an eye it retracts into the Neuron. When they move the make a sound that sounds like they're saying YOBE! They only appear in 4-4, Neuron Jungle.