These super cute bows are the perfect addition to a collar. They have two elastic loops on the back and should fit up to a collar width of 30/40mm.
Personally, I think these are too big for the average cat to wear comfortably for more than a photo - obviously this depends entirely on the cat. If you're planning to use them for a cat/skinnier collar, feel free to leave a note at checkout and I can adjust the loop so it shouldn't sag.