I always pictured Gargants as Elephants.
Naw.. they are definitely the old prehistoric sloths -Jim confirmed that for me when I was getting my book signed at DragonCon last week.
They are described as more bull-like but "considerably humpier about the shoulders" and cloven hooves. Though looking back the head description definitely sounds more like that thing than the oversized bull horns I was imagining.
The closest thing Ive been able to come up with for the taurga was some kind of prehistoric version of a Woodland Musk Ox with oversized rear haunches made more for leaping.
We were both wrong, I asked Jim this as DragonCon, (I lucked out at the end of a signing in Dealer Hall-no crowd and I could ask a few quick questions.) He said its a reptilian rabbit like creature thats based on a picture in one of the old Monster Manuals (he said thought it was in 2nd edition of Monster Manual).
As a side note, thanks to Reaper's Bones 4 kickstarter, in about 1.5 yrs I should have all the items I need to do a Codex Alera scene. -I can use the Cave Sloth as a Gargants, theyve got Terror birds I can used for Herdbane, and there are other things that I can use to make it look like a Marat camp.