Thus far I've been unable to find any canonical information on what powers the following races worship:
Medusae: I have yet to find any info about what power they worship.
Grippli: ????
Quaggoth: Can't find anything on who or what they venerate.
Yeti: Only info is from Dragon Magazine 347 which states that they consider Cryonax to be a god. Now, considering that most yeti are NOT evil-aligned, there must surely be another power they worship. (On Toril I'm guessing they also venerate Ulutiu)
Also can't find any info on what the Taer worship (Though considering their appearance, I'm guessing they primarily worship Demogorgon, and perhaps to a lesser extent, Cryonax)
As far as I know, the only reference to medusa religion in 1st-3rd editions was in Dragon #106, in Ed Greenwood's "Ecology of the Maedar."
"Medusae and maedar respect, but do not worship, Skoraeus the Living Rock (see Legends & Lore, Nonhuman Deities), and have a like attitude toward lawful evil deities and creatures. They will cooperate with lawful evil creatures such as orcs, kobolds, or even devils, for reward or security or (rarely) under duress."
This makes it sound like they're not religious, but have a respect for certain deities, including Skoreaus, and may have pacts with certain baatezu lords. Of the Lords of the Nine, the Hag Countess is specifically said in the Book of Vile Darkness to have medusa servants (she gave one of them to Baalzebul), though she has no cult. Mammon would be another likely patron, given his affinity for monstrous races like mind flayers and dragons.
In 4th edition, medusas are followers of Zehir, god of serpents and the night, if the medusa shroud of Zehir is any indication. Set is the obvious equivalent (and Zehir was called Set early in the development of 4e); as a lawful evil deity, Set would earn the medusas' respect, at least. I can see them forming alliances with temples of Set, or cynically seizing control of them behind the scenes. Those who ally with nagas or yuan-ti might similarly associate with the cults of Shekinester or Merrshaulk.
The official answer to the medusa question seems to be Skoreaus Stonebones (respects), Hag Countess (serves), and Zehir.
According to Dragon #324, grippli have a "simple faith in a nameless froglike goddess... depicted in sculptures of wood or stone as a bulbous frog with vibrant rainbow skin." Most other races believe she may technically be a forest spirit rather than a true deity, but she's a goddess as far as the grippli are concerned. Others suggest she might be a relatively benign aspect of Blibdoolpoolp. "Spider and snake creatures round out the grippli religion as demons and evil spirits."
James O'Rance designed an unofficial grippli pantheon long before this Dragon article came out. You can see it here. It's actually very similar to the pantheon vaguely described in Dragon #324 (including a froglike mother goddess and an evil spider figure); this is probably not a coincidence.
Monsters of Faerun says that quaggoths are divided between those who "follow the beast" and "those who follow magic," which sounds like religious sects to me. Dragon #265 says that quaggoths are led by thonots, psionicists who fill a role similar to shamans, interpreting omens, performing rituals, and tending to the sick. Probably references to "magic" in 3rd edition should be changed to "psionics." 3rd edition's Drow of the Underdark says that those who "follow the beast" are "virtually indistinguishable in nature and outlook from wild animals," though they're said to fear the wrath of their gods if they use items crafted by other races. Those who "follow magic," on the other hand, "improve on their innate strengths, which they see as a gift from their gods (though still primitive and elemental)." It also says "Quaggoth clerics are extremely rare and worship a primitive aspect of Nerull. The god of death's trickery aspect fits in well with the quaggoths' battle tactics," while some few are solitary spirit-shamans, communing alone with the spirits or gods who created their race. So Nerull is the only named quaggoth god.
According to the Monstrous Compendium Greyhawk Adventures Appendix, "Taer worship their own guardian snow-god, asking for good hunting and snow to hide within. Clans carry a crudely fashioned stone guardian idol to protect them. Taer believe the size of the statue relates to the magical protection bestowed upon them." This snow-god hasn't been named; because taer are neutral in alignment, Demogorgon or Cryonax seem unlikely (but certainly not impossible). Of the demon lords, the ape-demon Ilsidahur might be more popular than Demogorgon; Kostchtchie might be another possibility. In Greyhawk, Vatun or Telchur would be appropriate, but the reference says "their own" snow-god, which suggests other races don't worship the taer deity.
There's absolutely nothing on yeti religion in either the 2nd edition Monstrous Manual or "The Ecology of the Yeti" in Dragon #127. I'd be tempted to have them worship Ithaqua (d20 Call of Cthulhu, page 306). The taer could worship Ithaqua too. Cryonax is the only yeti god named, however. Perhaps the neutral ones sacrifice to him out of fear rather than devotion.
To sum up:
Medusae: Skoreaus (respect), Hag Countess (serve), Zehir
Grippli: Nameless frog goddess.
Quaggoths: Nerull
Taer: Unnamed snow-god
Yeti: Cryonax