Papilio agenor is a large butterfly (120 to 150 mm span) native to southern Asia that belongs to the Swallowtail family.
Papilio agenor was formerly treated as a subspecies (Papilio memnon agenor), but in 2023 was raised to species level.
Papilio agenor females are polymorphic and can take different forms, some of which have tails.

The male Papilio agenor is tailless, above deep blue to black.
On the underside, the anal angle of the hindwing has a red patch and both wing bases are red.
The female Papilio agenor has many different forms, with and without a tail.

1st, 2nd & 3rd photo: A female. My Son Sanctuary, Hoi An area, Vietnam, 30 December 2023
4th & 5th photo: A male. Ben Tre area, Mekong Delta, Vietnam, 23 & 25 December 2023