Collectinator
Scans your mini-pets and mounts and provide a check list for what you're missing.
Download for
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Wrath of the Lich King -
Cataclysm
-
The Burning Crusade
Wrath of the Lich King 
Cataclysm 
The Burning Crusade Addon Details
Expansion
Wrath of the Lich King Categories
Developers
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Provides methods for determining which mini-pets and mounts you are missing. Open up your companion window (pets or mounts) and click the scan button. This will open up another window which will allow you to see which companions you are missing, along with how to acquire them.
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