Mustrum_Ridcully
Legend
I welcome Rachel, our newest member in the thread necromancer guild! 

Pre-existing monster means that the player needs access to the Monster Manual and has a full stat block to account in addition to his own character. That's why 4E doesn't do it this way.1) Encounter & Dailies where you select a pre-existing monster (the higher the lvl, the more powerful) that is *autonomous*. It follows simple commands issued via free (or perhaps minor) actions from it's summoner.
It might be interesting to actually create at-will, encounter and daily powers that work off a summoned creature. Maybe a new multi-class only class or a special build option for some classes? *ponders*6) Additional spell powers which can enhance/alter the summoned monster as well as some fun Utility spells.