Why do stalkers appear with the undead?

Question

I may have a strange question but today it caught my eye, we have a "monster" stalker in Tibia, he usually appears in catacombs and other places with undead creatures, but why? According to the cyclopedia and beastiary, he is an ordinary and alive human because he is in the category 'human' and not 'undead' so maybe someone has a theory or knowledge why stalkers live with the undead and why the undead don't attack them anyway that they attack every other 'living' creature.

Comments

Stalkers are not the only living creature that is not 'attacked'. In the dungeons beneath the Isle of the Kings, Monks dwell together with Stalkers and undead creatures.
The weirdest mixture seems to be the Dark Cathedral, where renegades share rooms with various undead.

Answer 1

Stalkers have the ability to life drain like most undead creatures. They were also introduced when Edron was created in 2001 according to Tibia Wiki. I cannot find a valid source confirming this but I think it has to do with these two factors. 1) Appearing with the other new creatures in Edron making it a trend to appear with the undead. 2) Having the ability to life drain like most undead. 

This is all the official information I could find on Stalkers: 

Source: https://www.tibia.com/library/?subtopic=creatures&race=stalker

Stalkers are persons that are permanently invisible. They sneak up to their victim, and attack it suddenly. Some of these assassins have been spotted at Edron. They stab with their daggers and suck life energy out of their opponents, but if the first strike is not deadly, they are weak fighters.