I saw some provocative suggestiions here that Hadad-rimmon was a kind of Canaanite cult that mourned the fertility gods in a very noisy way (maybe similar to the weeping of the Tammuz). Or that were dealing with two different mourning events, or that it is the name of the Messiah. While all these are possible i do not see the need to resort to such speculative measures. Hadad-rimmon could've well been a respected person or leader who died and was greatly mourned by his people. He might have been a respected priest or just a popular leader whose name was Rimmon (see 1 kings 15:18 for the name Tav-rimmon) who added the name Hadad to his name to show his fealty and devotion to this god whom he felt he had a personal connection. Name change was common in ANE. For example the Egyptian king Amenhotep IV changed his name to Akhenaten and devoted himself entirely to the sun god Aten. And the bible itself is full of such names: Baal Zefon (Num. 33:7), Baal Peor (Deut. 4:3), Baal Me'on (Num. 32:38), Baal Hanan (Gen. 36:38) etc. In all these instances we find the combination of the name of the god Baal with another name, which possibly existed before the 'Baal' was added as a prefix. We also find the name Hadad-ezer in 2 Samuel 8:9. So we see that such name combo was widespread in the Levant.
In any case, there is nothing in the name that would suggest that were dealing with some mysterious cult or Messiah. In fact, the name is not unusual all in the setting and culture of Ancient Canaan, this. This was most likely a person that was named Hadad-rimmon that was greatly mourned by his people. Who he was and where he came from we don't know and we will most likely never know. But i do think that he is to be identified with the Aram-Damascus dynasty as most of the kings there were named Hadad (see this) and the name Rimmon also appears in the lineage.