Wow, thanks, @Cookie Chomp! Incredible that you found them online!
I had in fact come here to post that I had finally figured it out on my own, while sorting my cutters this morning. I actually have the complete set as pictured in your link, but had not understood that Mystery Cutter was part of the package, as I has presumed the reindeer's legs were the trunk of the 3-D christmas tree also included in the set, and as these cutters were mixed in a bag of random other 2-D cutters.
I'm really glad we found a definitive answer for this one! (Though am slightly disappointed it is not a dancing weasel... But now that I look at it, it could make a cute 3-D bunny for Easter, if I just shorten the legs a bit?
Here is the picture of the body and the legs :