The Caterbillar is presumably a dragon that is mentioned only once in the reference book, The Incomplete Book of Dragons, when describing the Caliban Caves where many dragons (particularly young ones) overwinter in hibernation. In the caves the Vikings notice:
There is absolutely no description of what a Caterbillar looks like. It is most likely not a simple misspelling of 'Caterpillar', as Caterpillars do not overwinter, nor typically go into caves. However, because the name Caterbillar is so similar to Caterpillar, perhaps this dragon is caterpillar-like. We can imagine a multi-legged, worm-like body.