While I don't think it's possible to store individual monsters in variables, at least without using Fork Options, it is possible to store Monster Groups. The number stored in the variable corresponds exactly as they appear in the Database.
First create a Common Event in the Database (F8), named "Monster Var" (or something similar). Set the "Trigger" to "Call". In the Events Commands, simply place the following commands:
<>Variable Oper: [0001:Monster Var] Set, Rnd [1-10]
This will store the Monster Group as a random number between 1 and 10, which means that if the random number is 8, the Monster Group appearing for the fight would be (by default) 3 Gorgon and 2 Basilisk.
<>Enemy Encounter: Normal, V
This calls the enemy encounter sequence, but the monsters encountered will be based on whatever is stored in the variable, Monster Var.
The call routine doesn't just have to be a Common Event. It can be an Autorun map event as well, so instead of setting fixed encounters via the "Map Properties", encounters can be completely randomized from map to map, using different random numbers or variables.