The Aka-shita (sometimes Aka-guchi) is depicted as giant fur-covered monster with a gaping red mouth and long drooling tongue emerging from a large black cloud.
Some Japanese word-play seems to be behind the depiction of this creature. Aka-guchi (赤口) can also be read as Shakkou, a day which, according to traditional Japanese cosmology (Onmyoudou), is said to be the least auspicous and most dangerous date on the Japanese calendar. Thus, the Aka-guchi, like the bad luck predicted for Shakkou, appears out of thin air to wreak havoc.
The name "Aka-shita" can be traced back to Sekien Toriyama's drawing which appears to be based on an earlier depiction of Shakuzetsu-jin (赤舌神 - literally Red Tongued Kami), the guardian diety of the "Red Gate to the West" - Red Gate, too, is written as 赤口 (Aka-guchi).
These five concepts and their readings - 赤口 Red Mouth/赤口 Shakkou/赤口 Red Gate/赤舌 Red Tongue/赤舌神 Red Tongued Kami - seem to have combined and evolved over time to create the Aka-shita as it was known in 19th Century Japan.
Also, drawing a creature with fur emerging from a cloud using only cross-hatching was haaaaarrrrrdddd. Not sure I pulled it off...
shadows and effects
epic cross hatching and shading dude