Some easy answers:
So basically what I have here is a monster that actually melts and dies in late spring?
The materials that make up monsters don't follow all the same rules as the regular ones elsewhere in your game. For example, if you find the corpse, you may encounter some "water" on the ground. This "water" is monster debris, and will not flow like real water. I think that the melting option is impossible.
All I know is that I also had a scorpion (with mandibles and a poisonous stinger... isn't that completely redundant?) made of salt, and it wandered in the water just fine.
Mandibles are mouthparts.
Now, some puzzle-solving:
I googled "dwarf fortress" "monster of snow" but had zero matches.
Alright, then we know it's a randomly-generated monster instead of a normal named one you can find on the wiki.
Sounds like a HFS creature somehow met a horrible fate.
Had a Forgotten Beast made of snow, it fought as any other before my warriors managed to break the tissue.
The one remaining randomly-named option is night creature. However, "night creature" is out since the description would include the word "night." Also, Forgotten Beast is out, because it doesn't tell you to "Beware its something-or-other!" (There is always one such line, right?) Thus, HFS monster.
When you zoom to the location of the corpse, is the thing deep underground? Also, as mentioned, are there combat reports?
--Rexfelum