Its called a ruff. They're frilly collars that people like nobles and royalty would wear back in the 16th century.Celestia and Luna's ruffs would be big, as befitting their height, like this:Celestia's ruff would be white and the lace on the edges would be gold colored.Luna's ruff would be black and the lace on the edges would be purple colored.For Cadance, her ruff would be smaller and a lot simpler, perhaps colored pink, something like this:Twilight's ruff would be the smallest and simplest of them all, as befitting both her status as the smallest (adult) alicorn and her bookworm-ism personality. Something small and plain white, like this: