User blog comment:Lord Loss/Discussion of the Week: Portable 3rd Subspecies/@comment-3062343-20120527020930/@comment-5043481-20120601165256

In the arctic creatures live for a long time but grow slowly, eventually reaching large size. The frozen agnator could be the same age as the volcano one but did not grow as big. Large ones would be rare as they die before they get that big.

Jasperism, the ice armour melting on agnator sub would cool it as heat from its movement is melting the ice in combat. This may mean that agnator sub has enzymes that work at lower temperatures and has antifreeze protines it its blood like some arctic fish. This would explain it burrowing regularly, to cool its body and prevent overheat from its metablolism. When calm, the ice armour may stay frozen for much longer.

If it has enzymes that work at lower temperatures then a large body, which would keep heat in, would be a problem as it would overheat. The smaller body would lose heat faster and return to correct temperature. This may be why it still has the fin, to cool the body above ground. The fin would presumably be useless when burrowing through permafrost unlike the regular agnatot which uses it to swim.