I think the immunity to disease is probably enough (odd that the normal otyugh doesn't have that), but I might go for a save bonus vs poison if Echohawk also likes it.
Actually, the Immunity To Disease was in the draft because I used my Otyugh Redux as the base creature rather than the SRD Otyugh.
I feel like we've used or written a "vulnerabilty to rust" SQ we could use here. Remember where that is?
We gave the Sentinel Eidolon such a SQ:
Rust Vulnerability (Ex): A sentinel eidolon is affected normally by rust attacks, such as that of a rust monster or a
rusting grasp spell.
The reason I suggested Con damage is that a rust monster's attack would be instantly lethal to the aurotyugh if the latter fails its Fort save.
Hmm, on second thoughts, how about having
rusting grasp work normally, and note that other rusting attacks affect the aurotyugh like the
rusting grasp spell (Caster level = HD of monster if no CL stated). Then a
rust monster's rust touch does 3d6+5 damage?
Like this...
Rust Sensitivity (Ex): An aurotyugh's metallic biology is partly vulnerable to rusting attacks. The
rusting grasp spell affects an aurotyugh as if if were a ferrous creature. Rust special attacks, such as the antennae of a rust monster, have the same affect on an aurotyugh as a
rusting grasp spell: 3d6 damage +1 per caster level (max +15) per successful attack. If the rust attack has no CL, use the HD of the attacker.