Sometimes people just don't have enough pet space.
Also, sometimes you'll see pets with what are referred to as loyalty stats in the pound. There are lots of features that reward you with stats for completing them, and naturally, these stats tend to add up the longer you keep a pet. However, it's normally strength, defense and speed that are elevated. (Occasionally health and level stats are given as loyalty rewards as well, but this is rarer. But it's possible that's where the level 10 pet you rescued got it from) Without the equivalent health stats, those stats tend to be worthless when it comes to battling and trading.
That's not to say those pets are useless. Actually, if you were to adopt a pet with stats like this:
Level: 1
Health: 1
Strength: 160
Defense: 160
Speed: 160
... and you intend to train it, all you would need to focus on would be catching up health (and level, along with other stats like magic and charisma if desired). It would be much quicker to stat a pet like this than creating a pet and statting it from scratch.
Sometimes you can find some great pets on the pound, so keep checking! And congrats on your pound find.