1281 pets
Withers
Binds when picked up
Use: Teaches you how to summon this companion.
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Want it | 
Neutral
Idles about. Shrinks over time and re-grows when near a body of water.
Pit-pattering footsteps; vocalizations (on-click)
Shrinks down in size and will drink from nearby water source to regain full size.
Apothecary Furrows is located within Cliffspring Falls in Darkshore. Note that he is an NPC that Alliance can kill for a quest item.
Alliance players must loot an item from Jor'kil the Soulripper after killing him for a quest that is given while on the Darkshore quest chain. This item will start another quest and unlocks Remembrance of Auberdine.
Upon summoning this companion it will be taller than a gnome, but over time it will shrink to the size of the Teldrassil Sproutling. If it is near a body of water while in its smaller state, it will emote "Withers absorbs the water, growing back to full size" and grow back to its original size.
Vs. 
Undead
Vs. 
Humanoid
Mechanical
50% from Aquatic abilities
33% from Mechanical abilities
A solemn reminder of what was lost when the Worldbreaker changed the face of Azeroth.
Got him today!
UPDATE: You can now purchase him from Beastmaster Nuqut and Beastmaster Tirren in Timperstep Outpost in Dragon Isles!
This is still possible to purchase on Horde chars in 2020/BfA - just need to speak to Zidormi to go to Darkshore the way it used to be, then go to the cave to purchase.
The H/S breed of withers feels perfect to me. Teldrassil Sproutling with B/B breed seems worse off. Ruby Sapling also has some useful, and more powerful, breeds. The abilities goes really well with sunlight, but then you need to bring it on another pet which isn't perfect. Without sunlight the tree-ant pets are not really that powerful. Good if you like the slow and steady approach.
Worth saving a stone for. This pet is a great sub for a Blossoming Ancient for those of us who don't want to spend real money for a pet. It's really handy against Kiryn in the celestial tournament.
If you have a horde char you just walk right into the cave and buy it. You can clearly see the cave entrance on the map, then just go in through the poorly hidden waterfall entrance.
Here is a list of quests to unlock
The Shatterspear Invaders, assuming you haven't done any in Darkshore up to this point. It's pretty much all the quests you would normally encounter, but I did leave out the optional quests.:
I thought you guys should know that ever since MoP came out, withers doesn't seem grow back to normal size when he is near/in the water.
Really worth the trip and I hope his animations get fixed soon :(
But since when has life been fair. :(
Thanks goodness it's a bug with 4.0.6-fix coming soon I hope!
Apothecary Furrows is inside Cliffspring Hollow (cave) located at 56.86, 31.28 in Darkshore.
For those on foot, the ramp up to the cave is located at 56.20,27.20.
Enter the cave, turn left and follow the ramp down, and take a left at the bottom of the ramp. He's up on the ledge, standing next to the table, to the left as you enter that part of the cave.
Tigerlily.