Yeah, and the Champion goes through a cave full of them in Dromund Kaas and kills a big badass at the end.
I think that over the years SW writers have made Terentateks rather unimpressive when compared to their past descriptions of being beastly strong and murderously dangerous to most Jedi. Then pre-end game Revan came along and solo'd two of them in Korriban, which can only mean two things:
a) Revan is stupidly skilled even before end-game and his 'rebirth' in DK.
b) The beasts are not that impressive. Impressive, but not as you put it.
I personally think it's b).
And of course we know Ragnos is powerful, the question is how powerful. It doesn't surprise me that he can mind-fvck a Terentatek. In fact, it would be lame if any top-tier Sith couldn't pull that off.