RNV5k has proven to be one of the more durable pets, and has great stats if you find a rare one, which isn't terribly hard.
It's geared against beasts, which There you almost always come across when fighting.
Having the early speed boost helps, and thankfully it's not it's main damage attack.
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The rabid bite can be tricky, and if not used careful, can bite you right back, but with it, I've been able to do massive damage to foes.
The ability to switch to a critter damage attack at 10 helps keep it versatile, and the extra plating along with it's repair capability helps in the long run as well, especially when going from one fight to the next, and not hitting up a stable master every 5 minutes.
Overall I think this pet has a good selection of skills available to it, you just need to adjust them accordingly to the fight you're going into.
This was one of the first pets I cranked to 25.