Les, adding to what pat and MK said, in this case there is the issue of whether to write it as one word or two, or perhaps hyphenated. I don't know how dictionaries decide which variant to list, but often they select one and don't list any alternatives. The online Macquarie has wood rat as two words, but I don't know if that is included in the compact edition.
Another factor is that some dictionaries are aimed at a particular audience. In the case of the Macquarie, its claim to fame is as a record of English as used in Australia. The woodrat (or wood rat) being a creature from North and Central America, the editors might have thought it was a term they could leave out of their smaller editions, as the word is very seldom used here in Aus.
Anyhow, as the one-word form woodrat is listed by some of the Oxford dictionaries including the online one, and it is in use, I'll add it as a rare word. I think the other name for this animal, the pack rat is somewhat better known, because it's also used in a figurative sense, for a hoarder. For some reason the online Oxford gives this as two words, without any alternative, but Dictionary.com lists packrat as a variant. So I'll add that as a rare word too.