GreenTengu
Adventurer
Well, I wouldn't really call them "monsters" if you properly flesh them out and give them actual cultures, personalities and general free thought and free will...
But you can pretty much take any of the humanoid or fey that are listed as "Lawful Evil" and any other humanoid or fey that is listed as "Chaotic Evil" and, so long as they inhabit the same environment, you can be pretty certain they will be in conflict.
Hobgoblins vs. Orcs is an easy one to do and the PCs can really take either side with there being no real "right" choice and you needn't much more justification for them to go to war beyond simply both wanting to prove their combat superiority and each other being better challenges than the weak humans and halflings about.
Kobolds vs. Gnolls could be done with the Kobolds being such underdogs that it is hard not to pull for them.
Drow vs. Rakshasa fighting over some powerful artifact is possible, though the MM gives such different power levels for the two that it will require considerable work.
But you can pretty much take any of the humanoid or fey that are listed as "Lawful Evil" and any other humanoid or fey that is listed as "Chaotic Evil" and, so long as they inhabit the same environment, you can be pretty certain they will be in conflict.
Hobgoblins vs. Orcs is an easy one to do and the PCs can really take either side with there being no real "right" choice and you needn't much more justification for them to go to war beyond simply both wanting to prove their combat superiority and each other being better challenges than the weak humans and halflings about.
Kobolds vs. Gnolls could be done with the Kobolds being such underdogs that it is hard not to pull for them.
Drow vs. Rakshasa fighting over some powerful artifact is possible, though the MM gives such different power levels for the two that it will require considerable work.