:there is a wiki article about Belus. If the name of Decebalus was a roman form, then it has to mean something like "Dacian King". If the name of Decebalus was a dacian form, then it had some connection with the parthians ( see the parthian god of war Bel Marduk - from babylonian)   
 :here an interesting article 1 ; although it speculates much, the symbolism on the dacian helmet of Cotofenesti and the symbolism in the bas-relief depicting the suicide of Decebalus on Trajan's collumn (allegedly worked by a syrian) worth reading it. 
 :As i know, Decebalus had an alliance with the Parthians. Take a look to the depiction of a parthian from Severus' Arch 2, and compare to the depiction of dacians from Arch of Constantine 3. i don't claim i can "prove" that Decebalus or his dacians were parthians, but I think the info regarding the relation between the name of Decebalus and the name of "Elagabalus" is objective... 

 *Again, what historical sources about the etymology of Decebalus are there?