Actually, if anything, the combo would be Belt + Tigre, as the name "El-Tigre" is from a animated show, and I'm pretty sure they're not making names that includes references to other animated media. That said, I only added the Bel part as a reference to "fire", which both Belanus and Belisama represent respectively, namely the sun.
Belphegor in turn, is a nice fit to the supposed Fire/Dark typing for Litten3's Wrestler form. Its namely in the Concept art that Litten3's evo is being lazy, sleeping, which may represent the sin of Sloth.
Of course, the latter is a big stretch, but if the name Beltigre were true, its reasonable that the name also includes some reference to fire. Bel is still a word after all.
Beltigre possible name construction (if true):
1. Bel - derived from Bel, or Belenus/Belisama, a celtic deity of the sun. May also reference to Belphegor, the demon of Sloth.
2. Belt - a band of clothing worn around the waist.
3. Tigre - Spanish for "Tiger".
Not really..."el tigre" just means "the tiger" in Spanish...is it a show? Sure. Doesn't mean it has complete rights to two Spanish words. With litten3 seemingly based on a mask wrestler (Tiger Mask), giving it a Spanish-sounding name fits. "El Tigre" is also a pretty fitting name for a luchador. Again, though, these might not even be real. However, I am pretty confident in terms of where the name is supposed to come from.
Your Belphagor theory is not bad, but there's some stretching. Considering that "bel" is a pretty generic word "part", it itself can't really mean much. I mean there's belt, belligerent, bell, Belarus, belabor, belate, believe, etc. Completely jumping to Belphagor (a proper noun of all things) seems a tad bit...farfetched...I mean sure it was seen sleeping, but that's probably to give some character to that specific beltigre. I'm not sure that's indicative of the species. Now if the "bel" was in komala's name, I'd be pretty inclined to believe that it may be a reference to Belphagor...
The thing about a pokemon's name that makes it good is that not only should it encompass what the pokemon is about, but also cleverly flow (This is why talonflame is probably the worst name ever not counting some first gen names). It's even better if there are multiple obvious meanings. My favorite one of the revealed 'mons, for example, is
Comfey. At face value, it is pretty simple. Then you realize it's a combo of "comfrey", "fae", and "comfy" (and maybe even "convey") all in one. These associations also don't really require a leap of faith to see either.
Bewear is also a pretty great name. Even the simpler names like bounsweet (bounce + sweet) and cutiefly (cutie-pie + fly) flow very nicely. However, they are just that: simple names. Is bounsweet a soup ingredient? Probably not. Is cutiefly going to shank me? I don't think so....
Some (actually most) of these leaked names carry this exact charm. Just to name a few, there's mightior (mightier + meteor + a progression from "mini"), goruggla (gorilla +rugby +ruggid), and that whole ice/bug line based on insect parasitism. My favorites though, are still "darline" and "darwisker". Darline is more simple appearing to be just (darling + feline), but when it evolves, darwisker implies the dar- is also from "Darwin", which corresponds to the supposed theme of this pokemon. -line is also a reference to "family lines" where genes are passed down from parents to offspring. These two names are really smart.
Whether this is real or not, the names definitely sound better and more "pokemonish" than the ones from that other leak. I mean "carphin"? what is that even supposed to be? Car + dolphin? Carve + dolphin? Carn- + dolphin? It's supposed to be a psychic type so none of those really work...