鈥淚f they duck to get away, keep their tail low or ears back, or generally keep their body tight and low, that鈥檚 submissive, appeasement behavior,鈥 Alexandra Horowitz, via Metal Floss.
Horowitz suggests watching for any signs of duress in order to make sure your pet is as comfortable as possible. If they鈥檙e unhappy with what they鈥檙e wearing, try to ease their stress by choosing a less constrictive costume.