Bats aren't really a threat and having to swat constantly huge clouds of them when wandering about is annoying. I suggest these be neutral since they probalby wouldn't attack large creatures anyway. Neutral bats would be better for ambiance and would reduce the Tendon item clutter that results from hundreds of them dying every reset.

The few (lol) hostile bats represent a minority of all the bats in the Underdark. There isn't really much of a reason for neutral bats since they'll just add to lag and not offer a lot in terms of ambience. We can't really make them larger, either, because the next suitable model in line is the black dragon wyrmling we use for the draghazar.

Or find a use for those tendons. They're actually a massively useful item.