This is a replacement cap for the rear wheel bolt of a Louis Garneau F-UN balance bike. You can show off your style by printing it in fun colours!
You can see this model (yellow) compared to an original (black) in the photos.
Update 2024-01-01: There is a variant of the model suitable for printing with flex filament (the snap clip has been adjusted). I found the regular versions to crack about as often as the original, but the flex one is near-indestructible.
Link: https://www.louisgarneau.com/en_us/catalog/product/view/id/118361
The author marked this model as their own original creation.