The trusty Monster Reaction Chart from T&T 5th Ed., section 2.4 More About Monsters.
"Not all monsters need to be hostile toward adventurers in their domain."
Monster Reaction Chart
2 Monsters and men automatically attack each other.
3-5 Monsters automatically and openly hostile.
6-8 Encounter may be friendly in a guarded manner, but either side will attack if provoked.
9-11 Monsters more interested in parley or making deals than in fighting. There must be language in common for parleying.
12 Monsters are panic-stricken and flee immediately.