This file is a resource file for a spell using effect 127 (summon monsters). When the spell is cast, a number of creatures listed in this file is summoned based on the power level defined in the effect.
These files appear to be leftovers from BG1.

In PSTEE we have (monsum01.2da):

2DA V1.0
0
     RESREF
1    koboldsu
2    tasloisu
3    xvartsu
4    dogwisu
5    dogwasu
6    gibbersu