Pingo is the imaginary friend to Chad. He looks like a cross between a monkey, a goat, and some other animals. He is very mischieveous, and does not like being left alone.
He may stay the same age forever, as is evidence in the book. Which means, even after Chad dies, Pingo may still be around. Or it is possible that Pingo will die with Chad, because technically Pingo is part of Chad.