RPG Insanity

Okay so there are some craftable items which mats need the Boss Monsters

e.g Cerberus for that one Fire Spell

So my question is, can you get those monsters again? How rare are they and are they Trap only?

As far as i know, according to the enemy guide posted somewhere, some of the monsters are capture only and can be repeated.  Cerberus should be one of those.  But I don't remember and am just referencing the guide.