Immediately makes me think of a combination of βέλ- and φονά / φον-.
My sense is that the idea of names like this referring to lost stories, potentially involving real people subsequently mythologized, comes largely from Frazer, and is a worthwhile speculation, however
My Greek professors emphasized the intentional meaning of names in Greek mythology. Confusion arises when we can't find Greek referents, as in the case of the first part of Cassandra (although I think Graves makes the strongest case for "engangler of men", based on the nature of her curse and the depiction of Agamemnon bound up with nets in the bath.)
The literal meaning of Bellerophon as some form of martial "darter" (per Pegasus) is so apt that it has to seriously undermine alternate hypotheses.
- I think it's quite possible belua has that form because hunting wild beasts often involved casting spears or shooting arrows, which is less risky that melee with the wild beasts.
But this must be taken as a soft answer, pending etymological connections to back up the semantic analysis.
The American Heritage Dictionary of Indo-European roots lists:
bhel-² "To blow, swell; with derivatives referring to various round objects and to the notion of tumescent masculinity"
That is not inconsistent with Bellerephon's role, in the sense that Pegasus may be understood as anti-chthonic, and on his flying mount he would surely come like the wind, but bhel-² may shed light on the beast connection via "bull". (The ancients didn't have the same detailed taxonomy of species as the moderns, so words for types of animals could be used generically. Thus, the root might have been used for anything "beastlike".)
There is also a relationship of this root, via "blow", with "roaring" [see: "bullroarers"]. This meaning seems to have persisted in words like "bellow":
bellow, verb, (of a person or animal) emit a deep loud roar, typically in pain or anger.
SOURCE: Oxford Living Dictionary
The relationship of wind/breath and roaring is not random, and Bellerephon can certainly be understood to "roar like the wind" when swooping to the attack atop Pegasus.