Etymonline has the original meaning of monster as
c.1300, "malformed animal, creature afflicted with a birth defect"
but I am curious to know the term used at that time -- and even earlier -- for (one of) its current meaning(s): "mythological or fantastic creature: dragon, fairy, brownie, griffin, etc.".
The same entry suggests aglæca as a possibility. Any further support (or conversely, examples showing this use to be poetic or otherwise unusual) would be most welcome.
Also of interest would be terms in other Indo-European languages (esp. Latin, Greek, the Germanic languages, and the Indic languages) before, say, 1000.