I mentioned these very briefly when talking about gnomes, but didn't go into any detail. Never seen one in a BX game. There was a 3e campaign where a gnome illusionist had one as a familiar, but other than that...
Ferret, Giant (from Moldvay)
Move: 150' (50')
Att: 1 (bite)
No. App: 1d8 (1d12)
Unlike the weasel version, giant ferrets are "only" three feet long and just over 1HD. In terms of randomly encountering them, they only show up in grasslands and -interestingly enough- inhabited lands. They never appear randomly in dungeons.
They aren't really that impressive in a fight. With their low hit dice, skittish morale, and single simple attack, they should be fairly easy to deal with. What's most interesting about these are two tidbits from the description.
"They hunt giant rats underground and are sometimes trained for this purpose."
This ties in nicely with them appearing randomly in inhabited areas. One could see folks not particularly interested in tangling with giant rats, so they send in these tubes with teeth to deal with it. Which brings us to the next point.
"Unfortunately, their tempers are highly unpredictable, and they have been known to attack their trainers and other humans."
This isn't anything earth-shaking, but it's yet another example of the plot-rich material just lying there in the text of Moldvay & Cook.
I can imagine a town mini-adventure involving some irksome giant rats, the ferrets that were supposed to take them out, and the rat-catchers that "trained" said ferrets and then lost control of them. The ferrets are wreaking havoc, as they have found there way into several homes and stolen people's valuables (ferrets will steal almost anything, especially shinies). The rats are still a problem (perhaps spreading disease), and the rat-catchers are desperate to clean up the mess. Enter our brave, low-level heroes who must now make a foray into the sewers to rout out the offending rodents and mustelids, possibly encountering something more sinister in the process.