I rate Darkfenne Bat very highly - it's in lots of my books, as a turn 1-2 play. It's cheap, but worrying enough it needs dealing with. Most books have a Purify or Wand of Healing, but it gets it out fast, so it's not there later.
It's my favourite creature for a Straywood pet, bar none (have it in about 4 books). You force them to overpay with two reasonably good attack spells, or they start taking 4 dice plus rot from something you paid 7 mana for...