A gray rhino event is a counterpart to a black swan event; while a black swan is an event we don't expect because it is so rare, a gray rhino is an event that we don't think about... but do expect. It is an elephant in the room, but one that is posed to attack.
Gray rhino events include things like supervolcanoes, asteroid impacts, and pandemics. They are large, scary things that we know are bearing down on us, but we ignore the risk because it's Just Too Much.
The term was coined by Michele Wucker, an American policy analyst and writer, who used it in The Gray Rhino: How to Recognize and Act on the Obvious Dangers We Ignore to refer to predicable political and economic disasters. It had previously seen only moderate use outside of these contexts, but with the event of COVID-19 it is starting to come into more general use.