The Stalker only ever appears on the level "Come And Take Your Medicine" in the Pathway Into Darkness demo v2.0. You'll only find out its name after you kill it and if you happen to die by its hands you'll get an amusing death dialog. And yes the Stalker looks and acts just like an Ooze except for its color.

The Stalker's death dialog does appear in STR# 1003 of the Pathway Into Darkness v1.1 resource file but since the monster itself never appeared in the full game or in the demo (v1.0) it could never be attributed to any monster until v2.0 of the demo.

For whatever reason the Stalker never makes an appearance in v2.0 of the full game but its death message was retained as was its name in STR# 2001.

Back in May 1994 David Weitz asked the following question on comp.sys.mac.games

From: vcsc5370@sfsuvax1.sfsu.edu (David Scott Weitz)
Newsgroups: comp.sys.mac.games
Subject: stalkers in pid-demo
Date: 14 May 1994 20:26:51 GMT
Organization: California State University Sacramento
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NNTP-Posting-Host: sfsuvax1.sfsu.edu


I recently played the demo for Pathways into Darkness and I encountered
a creature not found in the commercial version-
A stalker- Looks like an ooze, but weaker- It only takes 4 bullets to
kill and stops shooting at you if you stab it.  Does anyone know why
it shows up in this demo and not the commercial version?

Some two months later Doug "speedy" Zartman, P.R. Director at Bungie Software, replied:

From: bungie1@aol.com (BUNGIE1)
Newsgroups: comp.sys.mac.games
Subject: Re: stalkers in PID demo
Date: 19 Jul 1994 15:11:02 -0400
Organization: America Online, Inc. (1-800-827-6364)
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DW:
There are a lot of things that show up in the demo but not the game, and
vice versa.  The demo is made to let the player explore the PID world
without having to struggle with it too much - the demo is meant to be less
challenging, and more revealing of the way the game plays.  Some of the
things that show up in the demo were leftover monsters/items that weren't
used in the game.

Doug Zartman
P.R. Director
Bungie Software

Hmmm... left over items as well?