... you are not seriously thinking of dressing a guinea pig up, are you? :o
Its horrible for them... they cant move around properly, cant scratch themselves, its uncomfy and stressful for them to get them into one...
I mean with hats and stuff like that I have the opinion that if its just for a quick photo, its alright. It will just take max. 5 minutes.
But I think a coat goes to far, having to get the little paws into the sleeves.. sorry, but I think thats cruel to the poor piggy.
