Mobs can RARELY spawn with a random item/drop a random item on death, such as these examples from what I encountered:

A drowned that spawned with an ELYTRA.

A dog dropping Beetroot.

A Drowned dropping Blue Ice.

A cat dropping Nether Quartz.

None of these mobs picked their items up as dogs and cats cannot pick up items, and the Drowned with an Elytra did not drop the Elytra when it was killed. The beetroot could not have been related to the gifts cats give as it is not a gift a cat can give.

To reproduce:

Simply just play normally, and with a rare chance, you could encounter it.