Rhinoceros by Eugène Ionesco

"When a rhinoceros charges across the town square one Sunday afternoon, Bérenger thinks nothing of it. Soon, however, rhinoceroses are popping up everywhere and Bérenger's whole world is under threat. What will it take for him to stand up to the increasing menace of rhinocerisation?"

To make sure we're all working from the same translation, I recommend getting the collected Bérenger Plays, published by Calder:

Amazon Paperback
Amazon eBook
Bookshop(dot)org
Direct from the publisher