RTF is a global modification that replaces all towns in the game.

although, perhaps there was a secret eastern laboratory of Chimera mages, where they created yetis and were destroyed by them.

So many questions and so few answers... I still lean more towards the idea that this is not an artificially created creation of mages, but rather, a genuine representative of the fauna of Ashan. Although, I admit, the theory about a hidden laboratory of Chimera mages is quite interesting. Besides, nothing prevents the existence of several subspecies of yetis.