Ressan the Needler

Ressan the Needler is a level 9 rare white bat that can be found in Tirisfal Glades. He has a large patrol area that spreads from the fields of Cold Hearth Manor, past the Scarlet Watchtower and on to Nightmare Vale. He is pretty easy to spot when he is up because he is quite large and all of the other bats in the area are black. I have indicated his patrol area in red on the map below. 

He drops the typical green or white item and a fairly decent amount of experience for leveling characters.

He is also desirable as a hunter pet for low level hunters because there are so few bats of this color and he is definitely the easiest to come by. The only other tamable white bats are: Blind Hunter, the level 35 rare elite in Razorfen Kraul and the level 75-76 Zul'Drak Bats. Blood Seeker, the level 15 bat that shares a spawn timer with Aeonaxx in Deepholm is also white but currently only stays spawned in for a few seconds when he does show up.

Ressan the Needler can be found with NPCScan. You will need to add it, his NPCScan ID is 10357.

You can also use this macro to help find and tame him:
/tar Ressan the Needler
/stopmacro [noexists]
/cast Tame Beast

I'm not sure on his exact spawn timer, but before Cataclysm it was between 1.5-2.5 hours. I believe it is much shorter than that now, probably more like somewhere in the 30 min range.