Although it’s said that Philo of Byblos wrote the very first one, and that Amarasimha wrote one in Sanskrit called the Amarakosha, the first English thesaurus as we know it was written by Peter Mark Roget. As a writer, I use a thesaurus almost every day (at least), so I am very, very grateful he decided to do all that work.
Can you imagine how hard it must’ve been to compile all those words before the internet, phone, or even the telegram? Thank you, Mr. Roget!!!