Because I keep seeing screenshots related to people having Unicorn Stuffy in-game, I'll give some clarification as to how people have gotten it.
Various unobtainable customizations exists in BFN. Unicorn Stuffy (alongside all limited time and DLC customizations) are/were obtainable in other versions of the game. For the case of Unicorn Stuffy, it was only obtainable for one week if you logged in during the game's launch week during October of 2019. Despite the re-release of BFN on Nintendo Switch, there has never been an opportunity to earn any of the customizations on the platform due to the events giving them out happening before it's release.
There are mods/hacks on the Nintendo Switch version of the game that allow players to give themselves (and by extension, any account the modder would have access to) any item in the game. If you've ever seen someone with customizations such as Unicorn Stuffy, Pea of Valiance, and other weird customizations you may have never seen before, those were modded in due to no legitimate way of obtaining them.