The usual tune to those words is Orientis Partibus, which can be found in a number of church hymnals. The words "Conquering kings their titles take from the kings they captive make" were once set to it.

The original latin words (one version, anyhow) are:

Orientis partibus
adventavit asinus
pulcher et fortissimus
sarcinis aptissimus
Hez! sir asne hez!

T.