Sometimes you get lucky and collect an extra one. This is more likely to happen if you're a couple of levels higher in Monstrology experience than the creature you're extracting from.
I think if your spell ends the battle you also get a bonus animus. At least that is what I got from the reference book, though I haven't used monstrology while in a group. I do know that you won't get any extra animus if the monster dies from the tick of a damage over time spell, even if you had enchanted that spell with the animus enchantment. The critter must die from the spell being cast. Not sure how it would work with Minotaur as it hits twice in the same turn.