Hanuman, or Sugriva

Churning of the Sea of Milk
South wing of east gallery
Angkor Wat

Inclusion of a monkey god, as the anchor of the devas, takes us out of the Churning myth and into the Ramayana; the designers of the relief have added elements from the Ramayana into the original Churning myth.

In terms of the Ramayana, this figure must be either Hanuman, Rama's monkey general, or Sugriva, the king of the monkeys. If it is Hanuman, then (by symmetry) the anchor of the asuras would be Ravana, who is Hanuman's counterpart; but if this figure is Sugriva, then the anchor of the asuras should be Bali, who is the king of the asuras, as Sugriva is king of the monkeys.

The balance of this equation seems to be in Hanuman's favor, because only he had the strength to anchor the devas; Sugriva was a rightful but weak king, who needed the aid of Rama and Hanuman to regain his kingdom from the usurper Valin1. Freeman and Jacques identify the pair as Hanuman-Ravana; Mannikka and Roveda identify them as Sugriva-Bali.

Summary
Before leaving this relief, a few points may be summarized. Mannikka is surely correct that the relief as a whole can be interpreted in terms of astronomy and the solar calendar; her visual evidence of light and shadow at the solstices, together with the counted number of gods and asuras, is very convincing. Specific identifications of the large devas and asuras (diagram) are still debatable (2004):

Of the three, Freeman and Jacques have the weight of Ramayana tradition on their side (in the Ramayana, Ravana-Hanuman are much more important than Bali-Sugriva), and might be preferred, at least for the moment, for that reason.

Notes:
(1) However, Suryavarman himself gained the throne by overthrowing his great-uncle (Coe, p.116), so it is possible that he identified himself with the monkey king, who also had to gain his throne by force of arms.