I read the story Poison by Roald Dahl.
There is a possibility that there truly was a krait under the sheet. However, it may also be a figment of Harry's imagination because the krait did not bite Harry during the long hours it took them to check if it really is under the sheet.
Indian krait is a species of venomous snakes found in the Indian subcontinent. It is mostly found in the jungles but it can also be found in people's houses where warm places are abundant.