The description for Detect Magic doesn't explicitly cover this. Some monsters, like the Ankheg, are categorized as magical beast, but don't have any magical or spell-like abilities. If they are detectable, what school of magic would they be in?
I could picture some magical beasts having the aura of their spell-like ability. A Blink Dog might always emanate an aura of Conjuration since it uses Dimension Door. However even that seems like it would only have a lingering aura after using the Dimension Door ability.