Carrion bird vs Turtle vs Wind Serpant vs Owl (if I wanna make the trip to tedrassil).
Which one for leveling? And why?
Any one of Bat/Owl/Carrion Bird is useful because of Screech. Pet does more threat and takes less damage because of it.
Wind Serpent isn’t too bad and is a solid pet, it just isn’t the best option.
Turtles are the worst possible pet you could get. They have the lowest damage modifier, which means that they’ll struggle to hold threat. Their damage reducing ability (Shell Shield) further reduces their damage output. There’s no point in having a tanky pet if it can’t even keep aggro on the mobs it’s supposed to tank.
What is the difference between carrion and owl? Anything?
Carrion birds have more armor, owls have higher attack.
Check out Petopia.
wind serpent’s the best leveling pet, iirc
Get a boar. They’ll eat anything.
Ok, so here is how this works, do you want one to make leveling faster/easier? go Bat/Owl/Carrion Bird as they have screech which helps hold aggro on multiple mobs and is super useful when leveling faster.
Wind serpent is ok, and if I remember right eats bread, but if you like it, go for it, but dont expect it to be the best for fast leveling.
Turtles are tank pets, they have the lowest damage, but are able to tank the most amount due to shell shield ability. The problem will be the turtle keeping aggro however, so while it sounds nice, its not really the best, though again, if you like turtles, go for it, you can still level with them.
In fact, any pet you see you can level with, the choice comes down to do you want to level with a pet you like, or do you want to level with a pet that makes leveling more efficient and faster…
Yes, for one thing, the carrion bird can use both bite and claw, owls can only use claws. Now this is up to debate with hunters, but generally Bite is more rage/energy efficient for your pets, but claw does more dps. Personally I leveled with a bat, which uses bite only, and I found it to be amazing. Also pay attention to diet restrictions, that is super important.
This website contains all the info about pets, I HIGHLY suggest you check it out
https://www.wow-petopia.com/classic/
Owl or cat are generally the best. Differences are minor but for general leveling owl is the best of the screech pets and cat is the best of the pure dps pets.
Ultimately it is still probably best to just pick a pet you like.
The first quality you want is either Dash or Dive, or charge (boars). Any pet without one of these abilities is not very interesting in leveling IMO
Then, from my observation bite is better than claw to hold aggro.
My problem with any bird with screech is that in tight environments it is harder to make a surgical pull. It’s great to aggro several mobs, I agree, unless you don’t want to aggro several mobs 
So based on this, I always favored a cat for leveling i.e. dash/bite
Tallstriders all the way!
from What are the best pets early on around level 15-25? - #20 by Permagrin-faerlina
Boar
They eat basically any food.
Charge gets them on mob faster and root mobs for 1 sec.
Also, after the charge, their next attack is increased, resulting in more aggro.