Hrothgar, as defined by the opening of Beowulf, was a Scylding.
Then the task of keeping the strongholds fell to Beow, well-loved by the Scyldings. Long he ruled in fame after his father left the world, 'till in time an heir was born to him: the noble Healfdene, sage and warlike, who ruled the gracious Scyldings while he lived. Four children in succession awoke into the world from him, the chieftain of armies: Heorogar, Hrothgar, Halga the good, and Elan; I heard she was queen and dear helpmate of Ongentheow, the warlike Scylfing.
Hrothgar was the king of Sjaelland (Danish and found in Denmark). Historically it is known as being the oldest market towns in Zealand. The town was burned in 1290 by Marsk Stig and his followers.
Hrothgar's capitol was named Heorot (the name of the mead house he built in order to celebrate God (given Hrothgar was Christian)).
Therefore, the tribe Hrothgar ruled were the Scyldings of Sjealland. Essentially, Hrothgar is a Dane (meaning he is of Danish descent), as opposed to Beowulf who was Geat (the Geats hail from Sweden and are Waegundings (a lineage of Swedish warriors)).