Who is the most ancient historical Indo-European who is known by name?
In other words, is it Pythonos, king of Kussara (17th century BC) or are there more ancient figures?
Jared Diamond refers to "Hittite-like names in Assyrian," and there's a (possibly mythical) Hittite king from 23rd century bce, Pamba.
The earliest Hittite & Luwian (Indo-European, not Hattian) names date back to the 20th century BCE. They are known from the so called 'Cappadocian tablets' belonging to some Assyrian merchants near the site of Kultepe. Some examples of such names are:
Götze, Albrecht "Some Groups of Ancient Anatolian Proper Names", Language Vol. 30, No. 3 (Jul. - Sep., 1954), pp. 349-359