Symbolism, Imagery, Allegory
The rhinoceros as a symbol in a play called Rhinoceros? Um, duh.
Mass Hysteria
In the play, there is no greater symbol than the titular beasts that take over the town (and, it seems, the world). If you want to go all allegorical on this thing, you could say something like the rhinos are the Nazis or the Iron Guard (a right-wing extremist group in Romania, where Ionesco’s from).
In fact, in an article about the background of the play, Ionesco cites an account about the Nazis parading through Germany as one of his inspirations: “As they [Hitler and his entourage] drew nearer…the crowd caught up in a kind of hysteria […] the hysteria spread, and advanced, with Hitler, like a tide.” Freaky, huh? Basically, rhino = authoritarian bad guy.
Back to the play: better be careful with your comparisons, because this isn’t a “one-to-one” allegory. That is to say, characters in the play don’t necessarily represent specific people in history. In fact, it gets even more interesting than that.
So sure, the rhinoceroses can symbolize any tyrannical group that comes to power. Also, since you’re super clever and have fully tapped into what Ionesco is all about with this play, you realize that all the rhinos don’t just come marching in from a foreign land. They once were the people of this small town, and like the frenzy around Hitler, those people joined up and advanced with the group.
The rhinoceroses aren’t just symbols of a Fascist government, they’re symbols of mankind’s willingness to join up with groups bent on some form of destruction. In the end, it’s not an invading army that Berenger is howling against. It’s the people who used to be his neighbors, his coworkers, his friends, and the woman he loved.