Solve this riddle please

Use the riddle to name a correct animal that will gain you admission to a dance.
These animals will get you through:elk,yak,dog
These animals will never do; horse,mouse,frog


Would this dance be a costume dress- up?
Maybe it was a stupid way of saying come as a dog, yak and whatever a yak is.

Check with friends going. I think they mean come as one of the animals that will "do".

Good Luck!

The answer is that any animal whose name contains three letters can get you into a dance. Kinda silly riddle, huh?