The tentacle beast was a corrupted mutant plant, the result of years and years of corruption distorting nature, since in CoC1 more than 20 years have passed since the demons had appeared!
there would be no way for such a thing to arise in Savarra like that, since in CoC2, we are still at stage 1 of contamination, at most!
Unless it was created artificially, like Alraune, it's just that much more heinous!
Good luck coding this!
How about a "baby" tentacle beast, that starts spawning after defeatin Alraune?
Basically a slow, moving bush with vines as tentacles. It battles alone and despite being slow, it gets 2 turns and is tanky.