Mousey-Cho is a pseudonym for the Argonian character Heem-La from The Elder Scrolls III: Morrowind, (previously believed to have been named Heem-Lo due to the low resolution on the original Xbox, one of the game's launch platforms.)
Heem-La, or Mousey-Cho (as a select few individuals have come to know him) is a spellmaker, part of the Ald'Ruhn Guild of Mages. His popularity is mostly due to his ears, which seem to be significantly more mouse-like than those of most other Argonians in the game.
For reasons yet unexplained, the second syllable of this nickname has been adapted for use in a wide variety of ways, often meaning things that seem to be opposites of each other. The general application for the term 'cho' is usually an adjective meaning 'great' or 'awesome'; e.g., "This raspberry pie is cho." On rare occasion, it can actually be used as a derogatory noun; e.g., "That homeless guy is such a cho." 'Cho' can even be used as a verb at times. "Hey, stop choing around, you guys."
It is interesting to note that some people claim that since Mousey-Cho's services have been employed in the creation of several high-level destructive spells (such as the infamous BEAN GAS II and ELEMENTAL DESTRUCTION III ), he himself has retained knowledge of the spell formulas and is possibly able to cast them. Were this true, Mousey-Cho would have the capability to obliterate the entire island of Vvardenfell and most likely much of the surrounding territory, possibly even the entirety of the province of Morrowind. To date, no evidence has been brought to light to support these claims, but the Argonian has nonetheless developed quite the reputation as a master battlemage.