I saw on neilgaimanboard.com some one suggesting the following:
The "forgettable god" should be Mercury.
More explicitly, the "forgettable god" should be a Hindu deity named
Budha, the son of the moon god Soma and Tara, the wife of Brihaspati,
or Jupiter. Budha is one of the navagraha (literal nine-planets), or planet-gods in Vedic
astrology; to the ancient Roman, he is Mercury.
Much like the Roman Mercury, the Hindu Mercury is a god of two things:
wealth and the mind.
He governs commerce, both tangible and intangible; he grants wealth on
to his devotees; he removes obstacles in the way of success.
As the god of knowledge, it is Mercury's power to both grant and erase
wisdom. The "greatest among the wise," he has the ability to remove
thoughts from minds as well as grant inspiration.
The "forgotten god" is a dark-haired, dark-eyed, well-manicured man.
Budha had dark hair and dark eyes, and was considered to be the most
attractive of the navagraha.
Now is were we really get analytical:
The element mercury, also known as quicksilver, is the only metal that
is liquid at room temperature. It is notoriously difficult to contain
and is extremely quick to evaporate; the phrase "like trying to hold
mercury" is a common euphemism for things which are either easily
forgotten, or difficult to contain.
The "forgettable god"'s home is Las Vegas, which Gaiman goes out of
his way to describe as a place where money evaporates or melts away,
just like all knowledge of the "forgettable god."
If Mercury is both the incarnation of money and knowledge, and mercury
is also impossible to contain or retain, you get ... knowledge and
wealth that is impossible to contain or retain. The forgettable god.
The forgettable god wore what Gaiman referred to several times as a
"charcoal suit." The only thing that can contain or absorb liquid
mercury is charcoal, which is utilized in containers designed to
control mercury spills.
Wednesday orders the forgotten god a Laphroaig and water while they're
at the bar. Laphroaig is a single-malt Scotch whisky famous for its
peaty aroma, the result of a distilling process that involves
peat from the local peat bogs used to dry the malt. Mercury is a natural byproduct of peat.
Gaseous mercury is used to create neon lights, which Gaiman notes
numerous times are the hallmark of Vegas.
While I've already noted it, I think it's important to restate that
Mercury was the son of Soma.
Finally, the Hindu word for Wednesday is derived from Budha's
(Mercury's) name, just as our word for Wednesday is derived from
Odin's name. This may further explain why Neil made Budha the
"forgotten god" - because the book already had a Wednesday. (It also
explains Neil's use of the song titled Why Can't He Be You - think
about it for a minute - during the scene in the bar when Wednesday and
Mercury meet, and why Neil chose to describe the two in such similar
terms during that scene.)
I think this suggestion is the closest to what I have seen, so I am going with it.
I have found that the person that have stated the aforementioned has also posted the following:
I'll add that, upon further consideration, I think that "the
forgettable god" is supposed to be an embodied combination of all of
the different Mercurys: the planet Mercury, the element Mercury, and
the Hindi and Roman gods Mercury (who are virtually identical), with a
strong leaning toward the Hindi Mercury (which is why Wednesday can
bribe him with soma, and why he has dark hair and dark eyes).
The only Mercury that isn't consolidated into the forgettable god,
though, is Wednesday. Odin is the Nordic equivalent of Mercury. (For
more connections, as I noted, the Hindi word for Wednesday is derived
from Budha's name; furthermore, the French word for Wednesday is
Mercredi, which comes from the word ... well, I'm sure you can figure
Wednesday (Odin) is the only Mercury who isn't a god of commerce,
which is I think the reason why he is separate from the other Mercury.
He is, however, like all the other Mercurys a dispenser and dissolver
of knowledge, and by the end of the book you realize that Wednesday is
just as hidden and invisible as the forgotten god (both
metaphorically, because his true motives are hidden and he's
constantly conning people by hiding his true identity under a facade,
and physically, because at the end he's just a disembodied voice).
One of the dead giveaways that the forgettable god is Mercury (other
than the reference to Soma) is the way that Gaiman describes the
interaction between the god and Wednesday during the bar scene - he
writes them as warped mirror images of each other. Wednesday is "the
man in the light grey suit" and Mercury is "the man in the charcoal
(dark grey) suit." Wednesday drinks Jack Daniels, a Tennessee Straight Whiskey (legally defined as bourbon - disputed by distillers as it's own thing),
and Mercury drinks Laphroaig, a scotch whiskey. And, of course,
there's Why Can't He Be You. I don't think the lyrics are relevant,
but I think the song title is - I think it's a distinct reference to
the fact that Wednesday should "be" Mercury but isn't.
More important bits to note:
Because the planet Mercury is practically impossible to see with the
human eye, the Egyptian name for Mercury is Sbg, 'unknown.'
The conception of Budha caused a war of the gods according to to Hindu
Budha's consort, Ila, is a lunar goddess. Since Laura's last name is
Moon, we can assume that Shadow's last name is Moon, and because he
never knew his father Shadow would have had his mother's last name,
making his mother a Moon. His mother is described as being dark-haired
and dark-eyed, which roughly indicates that his mother looked
ethnically like the "forgotten god."