You’re not going to hear this word from a veterinarian, or see it in a biology textbook, but you can use “beastie” to refer to any kind of creature, mythical or real. The word has Scottish roots, and you might hear it used as “wee beastie,” with a jovial or even exasperated tone. When it’s not referring to critters, it’s used to refer to objects in the same sense that Americans might call something a “thingie.”