Ralphie is a stenopelhabbilis, and was temporarily the last living member of that race.
It was the offspring of the last pair of stenopelhabbilis who died in the Tokyo Sanctuary and were thereafter brought to the Old City Sanctuary. It hatched inside the freezing tube they were kept in, and escaped when Kate Freelander opened it to steal the parents' tusks.
As a small youngling, it roamed the Sanctuary and fed on everything it found, preferably hamburgers, although they were not considered healthy for it. Eventually, it found its way out of the facility and fled through the streets of Old City.
Henry Foss and Kate went out to find it and hunted into its nest in the sewers. It was there that they realized the creature had accepted Kate, the first being it met in its life, as its mother. Relieved, they returned it to the Sanctuary, where Kate named it "Ralphie", even though they did not know its gender with certainty.