Q:

What's "red leech" here?

Let me ask about the word I'm not sure about its meaning from The Golden Pince-nez by Conan Doyle.

As I turn over the pages, I see my notes upon the repulsive story of
  the red leech and the terrible death of Crosby, the banker.

What I want to know is whether the word "red leech" here refers to animal leech or a person. Anything can happen in Holmes universe so it could mean literally animal leech whose body color is red. But for me it looks a bit strange. I found the term "leech" could indicate a certain type of person which is.. a person who gives attention to someone over a long period in order to get their money or support, then the readers should take this word as so? That is the red-headed scum who may have killed the banker Crosby? 
The term "red leech" here is ambiguous even for native speakers too? Or is there any way you can judge which is which? 
Thanks.

A:

There is a reference to the red leech in The New Yorker May 24, 1952 Issue. 

According to Prof. John Percy Moore, of the Univ. of Peen., who is one of the worlds outstanding authority on leeches, this creature occasionall slips into the nasal passage and lodges in one of the ethmoid sinuses. Grows to enormous size there, and it's just possible it could kill a man. When withdrawn by forceps from the body on which it feeds, it would normally appear drenched in its victims blood.

Including its typos.

A:

It most probably refers to the Kinabalu Giant Red Leech

The Kinabalu giant red leech is a large bright orange-red coloured leech that is endemic to Mount Kinabalu, Borneo. It can grow to a length of over 50 cm.

Read all about it
Doyle was well-known for bringing exotic situations and animals to the Victorian masses. 
There's also a trope where Watson mentions lots of other adventures that happened while deciding which story to tell.  I expect this was another dramatically-titled adventure to add to Holmes' mystique. 
The death of the banker could be interpreted as another story, so Watson mentions

The repulsive story of the red leech, and 
The terrible death of Crosby, the banker.

