Found a stalker website, written in czech. Described something called a "TARK." I do not think this is a mod, but has all of the Beta monsters listed, like rats, rat wolves, dwarfs, and the tark... which is apparently a mutated horse.
The text describing it:
Něco jako zmutovaná podoba koně. Jehož tělo nemá již schopnost metabolismu a uzdravování tkáně. Chybí mu ocas, hlava je srostlá se zády a tvoří hrb. Přední končetiny jsou mnohem silměnší než zadní. Jeho tkáň je na mnoha místech porušena a místy jsou vidět svaly a kosti. Některé z kostí jsou zlomené a krev je hustá, ani skoro neteče. Váha je asi půl tuny a výška 2,5 až 3 m.
And the translation:
Such thing as mutated form horses. The body is no longer ability metabolism and recuperation tissues. It is missing tails, title is srostlá the back and form hvp. The front extremity are much silměnší than rear end. The tissue is in many places violated and the points are seen muscles and bones. Some of bones are broken and blood is dense, or almost be. Weight is about half tonnes and height 2,5 to 3 m.
The website is http://stalker.vokr.com/index.php?menu=nepratele
So why haven't I heard of these Tarks before? Why haven't I seen one in a mod? I need more info!
And just for the sake of reference, can someone post a bunch of screenies of Pseudo Giants for me, please? It seems Yahoo! and Google image searches have come up short!