I honestly don't see the problem with Girallons. IF they're lucky, and IF you stand still to fight them, you'll get torn a new one. Pretty much just like
Conversely if you keep on the move, they do pretty pathetic damage, have no real special abilities (rend doesn't really count, it's just like another melee attack), and are too dumb to really have any spectacular tactics. The only real problem is if they grapple you.
Frankly, I'd much rather meet a girallon than a tiger (HD 6, CR 4) - a tiger is much more likely to ambush you, and when it does, it gets to charge and then fire of 5 attacks.
Tendriculous - ok, so the regeneration is a bit of a pain unless you know how to overcome it (but it is easy to overcome - bludgeoning damage is everywhere). On the other hand, it can't hide, move fast or attack at range, and it'll be encountered outdoors. Really it's not much of a problem. Certainly not CR 9.
My votes are for:
Darkmantle - it's CR 1, and it has that lethal combo of darkness and blindsight. Level 1 characters don't have a counter to this (unless you allow faerie fire to work), and will be unable to find or hit the thing, or save friends once one of them gets grappled. At level one, even a grapple check of 0 is sufficient to grab a PC 50% of the time. When a PC's grappled, they will then have taken 2d4+4 damage, enough to kill the average non-fighter, and possibly enough to kill a fighter too.