What in the heck is a Hyalopterous Lemure? Well, aside from being an uncommon black creature from the Ice Age set, your guess is as good as ours at this point.
"Hyalopterous" is a fancy word meaning "having glassy or transparent wings."
Lemure entry from the old Monster Manual, circa 1979.
Ok, how about "lemure"? The word "Lemure" is quite obscure; it shows up on dictionary.com as "lemures" (LEM-yur-eez), which were "spirits or ghosts of the departed." A ghost with glassy wings… sounds good so far. "Lemure" is a questionable "singularization" of the word, and may have been copied from Dungeons & Dragons. The Lemure—a subtype of Devil—appeared in the original Advanced D&D Monster Manual. The idea of a type of lesser Devil seems to fit what the Lemure is in Magic terms: an evil black creature. This all makes sense, as the Ice Age team was pretty heavy into D&D, and the creature type was "Lemure," and not "Ghost" or "Spirit."
Of course, the poor artist—Richard Thomas—was not as well versed in minor Devils (or Latin), and painted what he thought the card was supposed to be… a lemur. That's right, a small, nocturnal primate from Madagascar with big eyes and a bushy tail. Who can blame him?
He did paint glassy wings on, so he got the hyalopterous part right.
Thanks to Timothy Thomas and David Welsh for their word knowledge.