Tsathoggua
Tsathoggua is an Old One described in several Cthulhu Mythos tales. Tsathoggua made his first appearance in "The Tale of Satampra Zeiros", in which his idol is described by author and creator Clark Ashton Smith. Clark Ashton Smith wrote the story in 1929, though it did not appear in Weird Tales until 1931. In the meantime, Howard Phillips Lovecraft wrote a story ("The Whisperer in Darkness") that invoked Tsathoggua which appeared in print a few month before "The Tale of Satampra Zeiros".
For those who want a short description, Tsathoggua is the Old One with traits of both a toad and a bat--he's sometimes described as looking like a giant furry toad. The description is actually a combination of how Lovecraft and Clark Ashton Smith described the creature, since each had quite different views on the subject.
Tsathoggua Ancient One
Clark Ashton Smith described Tsathoggua again in the 1933 story "The Seven Geases", where he described Tsathoggua as "batlike" with the look of a "sleepy black toad" and having great girth. Lovecraft described Tsathoggua as an amorphous and toad-like creature, though he later added in "The Horror in the Museum" that Tsathoggua was both black and looked like a "toad-like gargoyle" with "rudimentary feet".
K'n-yan - Tsathoggua Cult
The K'n-yan are a race that once worshipped Tsathoggua, until they learned about his true nature. (Strangely, they continue to worshipp Hastur and Cthulhu--among several others.) The K'n-yan live in a cavern underneath Oklahoma. They appear to look just like American Indians, but are powerful psioncis with advancement levels to understand and control atomic energy. The K'n-yan or Xinaian culture is first described in "The Mound" by Zealie Bishop (revised by H.P. Lovecraft). There it mentions a Spanish conquistador named Zamacona found the culture in the 16th century. The K'n-yan are mentioned later in Hazel Heald's "Out of the Aeons" and Lovecraft's "The Whisperer in Darkness". Another, more famous culture is associated with Tsathoggua worship, though.
N'kai - Tsathoggua Cult
Tsathoggua was worshipped in Hyperborea, where the god-being still dwells (in Lovecraft's 20th century). Tsathoggua is said to dwell in N'Kai, a lightless cavern said to be underneath either Yoth or Mount Voormithadreth. The K'y-yan visted this cavern in the distance past, but sealed up the cavern when Tsathoggua's formless spawn attacked their explorers.
Voormis - Tsathoggua Cult
The Voormis are the favored race of Tsathoggua, as they bear a close enough resemblance that they appear to be descended from him. The Voormis are described as being dark colored, three toed, and completely covered in fur. Their hated enemies are the Gnophkehs, which looked quite similar (though shaggier) than the Voormis. The Gnophkeks worship Rhan-Tegoth, a Great Old One. Both races have dwindled on a continent in Hyperborea, due to their constant strife. In fact, the Voormis eventually retreated to Mount Vorminadreth because their number had shrank so low.
Tsathoggua's Formless Spawn
Tsathoggua's formless spawn carry out the wishes of the great beast. These beings are quite dangerous in their own right. They are described as viscous, semi-liquid, and opaque. The black fluid is said to move by groping and pouring, though it can group short legs. These goo creatures are resilient, formless, and difficult to harm in any way. Also, they can take whatever form they need to attack their targets in just about any way they need to. Their attacks tend to include biting, crushing, or trampling their enemies. Furthermore, the formless spawn's substance was once described as so acidic that it would dissolve human flesh with the merest touch. Finally, these creatures can squeeze through the tiniest cracks in a room. By any description, encountering these creatures in a game should mean almost certain death.
Tsathoggua Family Tree
Because Clark Ashton Smith tried to create a complex and often illogical family tree for the Cthulhu Mythos creatures, Tsathoggua has a complicated family tree of which he's a member.
For instance, Tsathoggua is the grandson of Cxaxukluth, who is the "son the Azothoth" through the process of spontaneous fission. Apparently, Cxaxakluth lives on Yuggoth, but his progeny (or those which could escape) left his presence, because he's a cannibal. Ghisguth is the father of Tsathoggua, while Hziulquoigmnzhah is Tsathoggua's uncle. Tsathoggua also has children, though I'm sure the Lovecraft purists already see this seems to diminish these alien beings from the cosmic beings H.P. Lovecraft envisioned.
The Tsathoggua Cycle from Chaosium
Chaosium published The Tsathoggua Cycle in 2005, in which all the tales of Tsathoggua are told. This includes 14 different short stories by Clark Ashton Smith (5) and authors such as Ron Hilger, Stanley C. Sargent, and Gary Myers. Those wanting to read more about Tsathoggua can also read compilations of Clark Ashton Smith's Hyperborean Cycle. These tales combine Lovecraftian cosmic horror with stories of Iron Age kingdoms.
Tsathoggua Arkham Horror
Tsathoggua is an antagonist in the Arkham Horror board game. Tsathoggua appeared in the Dunwich Horror expansion. The creature has a doom track of 13 and a combat rating of -6. While it has no defenses, but anytime an investigator starts a battle with Tsathoggua, they must discard all clue tokens in their possession. The formless spawn of Tsathoggua gains a +1 toughness bonus, while investigators can't use the special ability of most locations on the board when he's active. Exceptions include Arkham Asylum, St. Mary's Hospital, and Ye Olde Magick Shoppe. When Tsathoggua attacks, investigators must discard 1 gate trophy plus 2 monster trophies or they're devoured.
Tsathoggua Merchandise
Tsathoggua is found in enough places that you'll find your occasional Tsathoggua merchandise. I've seen Tsathoggua t-shirts, Tsathoggua miniatures, and Tsathoggua poster prints. Bands like The Exoteric Order of Dagon and Seeser have written songs about Tsathoggua. So while Tsathoggua may not have the popularity of Cthulhu, the power of Yog-Sothoth or Azathoth, or the innature coolness of Hastur, the furry toad god has proven to have plenty of staying power among the gods of the Cthulhu Mythos.