Sherlock Season 4: 18 Easter Eggs & References From 'The Lying Detective'

Sherlock says that John called the previously mentioned Drearcliffe House case Murder at the Zoo. This is because it involved a Killer Orang-utan.

This is perhaps a reference to The Adventure of the Creeping Man, which involves an aged academic gaining simian-like abilities after eating monkey glands (yeah, it's a weird story).

It's more likely, however, that it is alluding to Edgar Allan Poe's The Murders of the Rue Morgue, seen by many to be the first ever detective story. Poe's detective, Auguste Dupin, was a direct inspiration for Holmes. In that story, the killer turns out to be a crazed orang-utan.