Updated.
Challenge Rating: 23-24?
They have 16 more HD and are one size category larger than the CR 20 t'uen-rin, which would normally be a net CR boost of +6, but lack the innate 20th-level spellcasting which makes quite an impact. They are also not as tough as the woefully-under-CRed tarrasque. They compare a bit more favorably with the CR 24 tayellah, although have quite a few less hp.
That seems a little high, considering they don't really have that much in the way of supernatural attacks or SLAs.
Consider how one would fare in a mano-a-mano against the SRD Pit Fiend:
Duruch’i-lin: AC 33, hp 432, Bite +41 melee (2d10+13) and 2 hooves +36 melee (1d10+6) or 6 magic bolts +36 ranged (3d6 force)
Pit Fiend: AC40, hp 225, 2 claws +30 melee (2d8+13) and 2 wings +28 melee (2d6+6) and bite +28 melee (4d6+6 plus poison plus disease) and tail slap +28 melee (2d8+6)
The "Uber-ki-lin" is better off attacking with its hextuple magic bolts (average 63 damage if they all hit) than its bite and hooves (average 47 damage if they all hit). The Pit Fiend is probably best off opening with
meteor swarm (average 70 damage assuming the Duruch'i-lin makes all its saves, which seems likely) then teleporting to melee (average 83 damage if they all hit).
Both have attacks almost as high as their opponents AC, so they're not likely to miss. The Pit Fiend does about 110-130% of the Duruch'i-lin's damage, but the Duruch'i-lin has 192% of the Fiend's hit points.
I'm thinking it's more of a Challenge Rating 21 myself.
These definitely need to go to Colossal, and probably rather quickly in the advancement path.
Advancement: x
A ch’i-lin is x feet long and weighs x pounds.
Oh yes, they need to go Colossal.
I'd prefer a slightly wider Gargantuan range than the one Freyar suggests, maybe:
Advancement: 33-42 HD (Gargantuan), 43-84 HD (Colossal)
As for size, shall we just upscale a unicorn and change the flavour to reflect the different sexual dimorphism?
A typical adult unicorn grows to 8 feet in length, stands 5 feet high at the shoulder, and weighs 1,200 pounds. Females are slightly smaller and slimmer than males.
Upscale it eightfold to put it on the Gargantuan/Colossal cusp would make it...
A typical adult Duruch'i-lin grows to 64 feet in length, stands 40 feet high at the shoulder, and weighs 300 tons. Females are noticeably larger and more powerfully built than males.
