One trope in tabletop roleplaying games in need of work is the concept of the encounter as a “one thing shows up and does one other thing.” There’s no reason why multiple monsters of similar alignments and motivations would not work together.
If these creatures work together more than once, they should have some sense of how each other operates as well as their strengths and weaknesses. Grouping like-minded individuals might turn what would be a typical encounter into something a little less predictable.
The following is one such encounter where two monsters, tactically minded and ruthless in combat, work together to create a small fighting force that would be dangerous to deal with, especially in a confined setting where collateral damage is something to consider.
As an example of this, I am presenting two races that fit the bill — fire giants and hobgoblins. Both love war, are tactically sound and would likely work in concert to achieve a goal. Both share the lawful evil alignment, so you can expect them to behave in similar fashions and seek similar solutions to the same problems.
In the correct environment to add complications, you have a problem that needs to be solved — quickly! The introduction of this encounter might well be a doorway to the start of something larger.
Encounter Type: Combat
Map: City encounter map
Lighting: None (flat battle map.)
System: 5E
Length: One shot
Installation Type: Addon
License: Roll20 End User License Agreement
(Personal Use Only, Do Not Distribute)