The bug

Fishing bobbers are entities but cannot be summoned. You can tell they are by looking in your debug screen when it is just you and a fishing bobber in the world. It will say there are two entities.

Steps to reproduce

1. /summon fishing_bobber

 You get an error: "Unknown entity: minecraft:fishing_bobber"

Additional Information

This might be considered a feature request but being that you can summon leash knots (for some reason) and that MC-30568 was valid this should be considered at the very least.