It's a deeper question that it may seem at first glance. There are worlds where I could just go and throw them in.
Because there, they would not alter the mood.
Innocuous and even beneficent, they would fit in worlds where the monsters are frequently marvels and wonders. In a world where everything else is hostile -- drakes, wolves, woodwoses, hags, ettins, dwarrow, all taken from a common folklore -- they are an anomaly. Also, they are the first actual shape-shifters if I use them. (I could throw in werewolves, were I so minded, but I haven't yet.)