You make good points guys, but I did promise putin an answer to his question if he answered mine, so I'll have to briefly derail again. Sorry.
@putin,
Thank you for answering my question directly.
To answer yours: there are various theories for why the LXX translators rendered the word as "unicorn." One is that they may have made a mistake. Sometimes the meanings of old words and names are lost. There are old names in English that we can no longer identify at all, like the Battle of Mons Badonicus. Though it refers to some actual place, we have no idea what.
Probably more likely, the translators utilized convenient Greek concepts of the time, for reasons of their own (perhaps the animal was unfamiliar where they were writing in Alexandria). As Invictus has pointed out, "wild goat" was elsewhere translated "Satyr," thought Satyr is Greek mythology. In that instance, it's clear the Hebrew word really does mean "wild goat," as it's used elsewhere in the Bible, so we do know that the LXX translators sometimes used very fanciful translations for animals.
A key point to remember: _The Bible actually refers to this animal having plural horns._ For some rather obscure discussion of this point, see here:
http://www.zootorah.com/VirtualTour/oryx.html
(The page's argument that the animal was the oryx seems interesting but not, to me, that compelling).