One of the interesting features of Richard Adams's novel Watership Down is his invented language "Lapine" spoken by rabbits in the story. Mostly, of course, the rabbits are shown speaking in English, but we do get a fair few words of their vocabulary (hrair, thlay, yona, pfeffa, and so on) as well as a few hints at grammar or at least word construction (e.g. the usage of -rah and -roo as suffixes). Some editions of the book contain a vocabulary list at the back, although I think the edition I read as a child didn't have such a list printed in the book, being equipped instead with a handwritten list provided by a family member who'd read it before and left a sheet of paper inside the book.
Was this language ever developed beyond a short list of words and phrases? I know Adams wasn't an accomplished linguist like Tolkien, but he seems to have put some thought and effort into creating at least pieces of a fictional language which wasn't strictly necessary for the storytelling. I'm primarily asking about whether Adams himself designed more of the language than is shown in the novel, but would also be interested if there's any fan-made extension, a fuller Lapine language designed by others.