For sentimental reasons, I wanted the Mulgore Hatchling to be awesome. He starts at uncommon and requires a stone to be rare, which is the first big knock against him. The second issue is that he's a critter who does strong anti critter damage. While there are a lot of critters out there he was strong against, there's also a ton of beasts, usually grouped with the critters. I enjoyed his shield, and eventually his feign death, but he's been underwhelming. The first critter stone I got went to him and he's still one of the two most likely pets I'll have out and following me around, but it's rare he sees a battle these days.
The pet I was most disappointed in, however, was the Frigid Frostling. I needed some elementals to take out mechanicals on a tamer team in Southern Barrens, so I leveled my Searing Scorching, Frigid Frostling and Pebble to level 10. Leveling them to 10 was my first clue. The scorchling kicked serious butt, mechanical or not, but the frostling could barely take out anyone, and when he did, it took forever. I left the frostling at level 10 and took my scorchling on to dominate tamers. He's a staple on my Major Payne team, the Darkmoon team, and anytime someone has a mechanical. The frostling, not so much. I don't even use him as a mini pet because the snow showers from his snowball Never Go Away. I was dumbfounded when I saw the tamer in Townlong Steppes was using a frostling.