Depends on what monster you are referring to and what you consider a monster in the first place. Some have looked at their own parents as monsters....That is a very vague question. Impossible to answer.
the first recorded Loch Ness Monster sighting!
That was prior to written history so there is no record of the first sighting.
Not every zombie sighting has been recorded, but some of the recorded ones go way back before the 1800's.
Chessie, the legendary sea monster, is said to live in the middle of the Chesapeake Bay. Chessie's first reported sighting was in 1943, with the last sighting in 1997.
First sighting reported 1933.
The Dutch
the first reported sighting was in 1933.
The first recorded sighting would probably be by Alexander the Great 329 BC.
Loch Ness, ScotlandAt Loch Ness by St. Colombus
We don't know if Nessie was really "seen," as real existence has not been proven. The last claimed sighting was in 2007.
The first recorded sighting of the northern lights dates back to ancient times. It is believed that indigenous peoples in the Arctic regions, such as the Inuit and Sami, have been witnessing the phenomenon for thousands of years.
The earliest sighting is widely believed to be St. Columba in the 6th Century CE