well it depends on how your game is set up.
if you have all your enemies off screen for example - in "standby" mode, and they were in a family called Enemies. You could "Pick Random Enemies" which would pick one of them and you could execute your logic from there...
Another way would be to make a unique animation or frame for each enemy in a single sprite. You'd give it an instance variable called "Type", and that would be which enemy it was.. that would correspond to which animation/or frame you would use.
Another way that might be best is to just have a function that spawns different enemies based on what you pass to it. So if you had 3 enemies: Imp, Guard, Troll
Call Function SpawnRandomEnemy(choose("Imp",Guard","Troll")) //this will pick a random one
If Function.Param(0) = "Imp" then do Spawn Imp
If Function.Param(0) = "Guard" then do Spawn Guard
If Function.Param(0) = "Troll" then do Spawn Troll