There are currently only 3 different ones, and they are áll common. (the unique one is easy to find at any given moment)
I have a feeling blizzard isnt showing much love for the sporebats, because the players aren't either...
So let me explain to you first why I love mine so much

Even though they seem simple, they have the most amazing animations
examples:
A normal attack where they Spin! (If he uses it last, he comes back to you spinning because it was still in that animation, priceless)
A normal attack where they make a somersault :O ('nuff said)
and his special spore attack animation where he just waves the smoke with his Flappy things. (It looks most like Dumbo the elephant trying to fly)
As for the Ability's effectiveness, it comes out handy in many situations, you will notice as you have only "just" survived that enemy mage in pvp because his fireball wasnt ready yet, or big flocks of mage creeps in dungeons.
I love the entire feeling around Zangarmarsh and its Sporebats, its hard to describe, "Soft and Mystical" would get closest to describing it.
I hope this will make some of you give the Sporebats a chance, and have a look at them, because I knów 90% of you hasn't

I Recommend taming the Blue one, its most beautifull, a good alternative would be the brown one, its a matter of taste, I dont like the green one.
Happy Hunting!
~ Griffley
PS: Any information about sporebats with visual effects in underbog etc will be rewarded with eternal gratitude.