In the novel Hallowe'en Party, Dr Ferguson is a physician with a practice at Woodleigh Common, and one of the few villagers who had been residing there for a long time.
Ferguson was the attending physician of Mrs Llewellyn-Smythe who died of heart failure some eighteen months before the events in the book. Her death was not unexpected as her condition was known and she was also careless about following her doctor's advice. Even so, according to Superintendent Spence, Ferguson had been surprised at the suddenness of her death as he had expected her to live longer.
Poirot makes a call Ferguson as part of his investigations into the death of Joyce Reynolds. Surprisingly, they do not discuss Mrs Llewellyn-Smythe. Ferguson, described as a sixty year old man of Scottish extraction with a brusque manner, would not speculate on who might have killed Joyce. However he goes on to add that it must be a mentally disturbed person who had gained access to the house (quite easy), there being so many such psychiatric killings going around. He adds that he was actually at the party himself--he dropped in late to see how it was going on.
Ferguson has a partner named Worral in his practice but he is the physician for the Reynolds family.