Boa constrictor
Richard Lancelyn Green, the editor of the 2000 Oxford paperback edition of "The Adventures of Sherlock Holmes", surmises that Doyle's source for the story appears to have been the article named "Called on by a Boa Constrictor. A West African Adventure" in "Cassell's Saturday Journal", published in February 1891. In the article, a captain tells how he was dispatched to a remote camp in [West Africa] to stay in a tumbledown cabin that belonged to a Portuguese trader. On the first night in the cabin, he is awoken by a creaking sound, and sees "a dark queer-looking thing hanging down through the ventilator above it". It turns out to be the largest "[Boa constrictor]" he has seen (more likely a python because there are no boas in Africa). He is paralysed with fear as the serpent comes down into the room. Unable to cry out for help, the captain spots an old bell that hung from a projecting beam above one of the windows. The bell cord had rotted away, but by means of a stick he manages to ring it and raise the alarm.