Roman history provides us with two more examples, one of which is fictional and the other reasonably trustworthy. Quoting the wikipedia entry on Spolia Opima
The spolia opima ("rich spoils") were the armour, arms, and other
effects that an ancient Roman general stripped from the body of an
opposing commander slain in single combat.
For the majority of the city's existence, the Romans recognized only
three instances when spolia opima were taken. The precedent was
imagined in Rome's mythical history, which tells that in 752 BC
Romulus defeated and stripped Acron, king of the Caeninenses,
following the Rape of the Sabine Women.[...] The third and most historically grounded occurred before the
Second Punic War when Marcus Claudius Marcellus (consul 222 BC)
stripped the Celtic warrior Viridomarus, a king of the Gaesatae.
So we have Romulus and Marcellus. I skipped the third guy because careful reading and cross-comparison shows he was not consul at the time he achieved his feat and so not (even co-) head of state.