A barefoot, young girl with a wooden pail of warm milk walks in a dangerous place. Oddly unharmed, she says to the party "All worried and weary come drink from my pail. Come drink this warm milk and feel hearty and hale."
Drinking the milk will always heal the drinker (2d6 hit points) but other effects depend on the kind of milk it is. The milk in the pail actually comes from a local denizen, so it might be unicorn, dragon, or ogre (don't think too much about the specifics it's a fairy tale).
Refusing to drink from the pail means the next encounter will be with something that has. (This will be obvious because of the empty pail nearby or milk dripping from its chin.)
So, this monster isn't really ready to run out of the box. You'll need to decide what the milk of each of the creatures on your random encounter table would do (if an ogre drinks dragon milk? If a unicorn drinks kobold milk?). If players decide to drink the milk, have them roll on your encounter table what kind of milk it was (it seems funnier and creepier to know you just drank rot grub milk than to just feel the effects).
Also, I think this milk maid would work best with some foreshadowing. Have the local village peasants arguing about whether you should or shouldn't drink from her pail with examples of what happened when their second cousins did etc. That way, even her appearance is a kind of an event.