It had to get away.
Instinct was a harsh rider, forcing it on even as its legs shook and it lungs collapsed. Sometimes the fear in the beast's muddled mind coalesced into a predator chasing it and it sped up, hooves stumbling over rocks and antlers catching on new trees which were more numerous as the climate became milder, although it was cold now and they dropped snow on the animal's back. But mostly it knew why it had to get away; it was a danger to the herd.
Sickness not so much lurked inside as oozed out of the beast, all fluids more stained with blood than they should be, loops of drool and phlegm swinging from its long lips. Evolution deemed a lonely death a safe one, and so it was compelled away from its herd, its females and feeding grounds and safe high ground lookouts.
It plodded, then stumbled, front knees knocking together. Half blind in truth it carried on, the destination unimportant compared with the distance.
Then it fell, straight into a water-filled crevasse. Underneath the brittle plate of ice it was deep, swampy. Survival instinct regained the upper hand at such a shock and compelled the animal to keep its head above water, to scramble at the sheer rocky sides with its hooves and knees until they bled. There was no chance.
Any energy it had left after its pilgrimage and sickness was sapped quickly by the cold water. With a last desperate breath in through its nostrils, a final squeal for its herd to save it from all of this, it went under, the crevice deep enough that it sank a few inches before its hooves could touch any bottom. It was already dead.
The film of ice reformed almost immediately using the old shards as starting points, and lacing out across the surface. It thickened, and snow covered it flake by flake, then layer by layer, piling up and pressing down. The sun swept across the sky, again and again and again, even the longest days and nights on the planet passing in a blink.