That's pretty close to the version we're already working on if its size were increased to Gargantuan, except its Con is higher and its bite is weaker - the current "enworld conversion" version has a 3d8 Bite at Huge, so it would have a 6d8 bite if it was Gargantuan with Improved Natural Attack (bite).
We try to stick to feats that are in the SRD, which excludes Improved Toughness. You can always throw another point in Con instead.
Also, Improved Natural Attack is not really necessary - you can just arbitrarily give it a 3d8 bite and use that feat for something else too.
I like Icewalking though, I think we'd better pinch that idea.![]()
I used the Monster Manual's suggesting that a Gargantuan creature's bite is at 2d8. Seeing as how its bite is its main attack, and it hunts, and it needs to contend with other larger monsters, I gave it INA for its bite and so it increased to 3d8.
Yeah I realized Improved Toughness is not SRD, so it'd have to be replaced with another feat.
As for Icewalking, it's based off the white dragon and a few other cold creatures in 3.5e. In the description of the tirichik, it specifically states it can move about ice as if it were walking on solid ground, so if that wasn't a nod to Icewalking ability, I dunno what is.

Yeah it's Gargantuan only because that was it's size in 2e. I should've compared its average width and height to the 3e version of sizes, it was probably meant to be a Huge creature in 3E. I'll have to check it.