The UnDeer was a Christmas time character for 7-Up, sometime in the 1971 period. It was a spin-off of their UnCola theme.
The UnDeer was a dude in a reindeer costume, sitting in a chair...
"Hi! I'm the UnDeer! How's your Hoofieees!?"
Information regarding the UnDeer on the net is sparse (most results come from a misspelling of "under")...Wikipedia has nothing (*see update below)...
OMG, someone has found a UnDeer tv ad. (December 2010)
[Update: a major breakthrough has occurred in UnDeer research, see bottom of article]
A new search has revealed this from eBay:
As late as July 2010, the Undeer remained relatively unknown as this posting indicates;
Undeer Christmas mascot
What year(s) was your Christmas mascot the 'Undeer' and is there any information available about this terrific mascot?
The Uncola campaign started in 1967 and through at least the mid-1970s. We were not able to locate a specific date for this particular piece of advertising that the reindeer appeared on, but think it is probably early 1970s. There are not many examples of this reindeer in the 7UP advertising pieces we have and so we hesitate to say that it was the Christmas mascot of 7UP during the Uncola campaign.
The net is filled with a post here, a post there from people such as I, claiming to have spotted this mythical beast long ago.
MacDonald & Associates' Television Commercials Mixed Ads 51-60:
#56) 7 UP: reindeer says he is an un-deer and loves this un- Cola
"Does anyone besides me know what an undeer is?" - Homesurfer, Professional Contrarian 2006
"I also remember the voice as being more like Beauregard, the janitor dog on the old Muppet Show" - rodpie 2008
"...but my favorite was the 7-up un-deer commercials. But for the life of me I can't find copies of them anywhere. - Ratt1959 2008
"I remember the UnDeer and he spoke French!!!!" - Carmen 2009
"The UN-DEER is an elusive creature last seen in Canada in the 70's on 7up commercials. has anyone seen him? - UN-Deer Hunter 2008
This last note may be a clue to why there is nothing on the net about the UnDeer...if it didn't happen in America, it didn't happen...[although the audio clips below seem to indicate the UnDeer was known in the U.S.]
The voice was provided by Paul Frees, who also was the voice of Disney's Ludwig Von Drake, and John and George in the television cartoon series The Beatles, among much other work (Craig at WFMU says: "Yes, Paul Frees did both George and John. He also did Boris Badenov, Inspector Fenwick on Dudley Do-Right, and such commercial voices as Toucan Sam and the Pillsbury Dough-Boy" Read more here.).
The voice resembled Marvin the Martian of Warner Brs. fame, which such catchphrases as "Wrong-O!" and "Hoofies!"
Yes Virginia, there really was an UnDeer.
*Update June 2009: Wikipedia doesn't believe there was a UnDeer, as an article on this very topic (submitted by yours truly) was rejected as "an obvious hoax"...
*Update September 2009: Gregory [My original tape was a dub from the broadcast cart at WLAP-AM in Lexington] has sent along audio proof of the Undeer's existence! Actual 1970's radio ads. Take that Wikipedia...