All Pets Allowed

If you level one or more pets to lv.25 before getting the lv.10 equivalent achievement, your progress on this one will be kept but hidden. Once you earned the lv.10 achievement, criteria that you already fulfilled will be displayed here.

Also, for any wild pets above lv.22, the level will be reduced by 2. Also, you cannot simply buy maxxed pets off the auction house to meet the criteria. You HAVE to 'hit' lv.25 yourself to gain credit.

Just found out it's possible to get this achievement without getting No Favorites. Only leveled one pet from 1, everything else was captured above 10. When you click on the achievement it looks like both are completed, but No Favorites is also found incomplete below in the Achievements>Pet Battles>Level category.

There is a total of 10 pet families (Aquatic, Beast, Critter, Dragonkin, Elemental, Flying, Humanoid, Magical, Mechanical and Undead), so this achievement requires you to raise 10 pets, 1 for each family, to level 25 (the maximum) through pet battles.

Credit is given for each family of which you raise a pet from level 24 to level 25. This means buying a caged level 24 pet, or capturing a level 24 wild pet (which will be weakened to level 22 after capture) is the most efficient method for getting this achievement.