Source: Waterdeep - Dragon Heist

Wondrous item, rare (requires attunement)

This bright plume is made from the feather of a diatryma (pronounced dee-ah-TRY-mah), a Large, colorful, flightless bird native to the Underdark. If you use an action to speak the command word and throw the feather into a Large unoccupied space on the ground within 5 feet of you, the feather becomes a living diatryma for up to 6 hours, after which it reverts to its feather form. It reverts to feather form early if it drops to 0 hit points or if you use an action to speak the command word again while touching the bird.

When the diatryma reverts to its feather form, the magic of the feather can't be used again until 7 days have passed.

The diatryma uses the statistics of an axe beak , except that its beak deals piercing damage instead of slashing damage. The creature is friendly to you and your companions, and it can be used as a mount. It understands your languages and obeys your spoken commands. If you issue no commands, the diatryma defends itself but takes no other actions.