This is a possible occurrence in survival, however here is a command to recreate the scenario attached in a text document because I hate the way mojang's bug reports keeps auto formatting the command.

You can summon a villager for it to attack and after the trident strikes it's target (or a block), it will return to it's rightful owner.

Unfortunately, the drowned cannot pick up the trident for... minecraft reasons; The trident thrown will instead continue to constantly hover around the eyes position of the drowned, eventually spastically spinning once it reaches a certain threshold.

I'll call this discovery, the Drowned Urchin.