This is not a guide for how to raise ur pet grade, but a guide on how to judge a pet.
I will try to cover as much as I can.
Below is a pet held by a ascended 150 modo
Look at the attack, its owner won’t have that much attack. This is a super extrem example for pet. Don’t copy it unless you seriously want to spend so much.

This is a what they called True 18(or 20) Grade Pet
btw, how much do u think it cost? 10s of thousands RMB(1rmb=1cs gold in ZHUXIAN)

—>Most important aspect for a half raised pet. Is how the growth is distributed. As you might know, grading on pet only depends on the total growth, In which All resis growth is totally useless, and the 2 most important growth is ATK and ACC. A True 12 grade pet should only have growth on ATK,DEF,ACC(OR at least ALL RES growth is around 1000) A true 18 Grade Pet can only have an average of around 2200 growth on All RES growth. Other wise its called Fake 12 or 18 grade pet simply because its much much weaker than the true type.(Not been harsh here lol, thats how they do it)But then most ppl in ZHUXIAN stays with a true 12 grade or true 16 grade or a fake 18 grade. Er fake 12 grade is kinda useless, but it still helps. Don’t get discouraged >.>

—>Celebean feeding undergoes a chance. And it follows the following rules (from reading, not rich enough to test it):
Less and less chance when the current growth is going toward its max
Chance is incredibly low when current growth is 4000
Less chance when the pet past grade 9
Less chance for NONE RES growth when pet past grade 16.

—>Pet skill slots are fixed. Its born with 3 skills, then it stays with 3 skills for life. You can use skill jades on it, but it randomly replaces the old one (Star Signature skill is unchangable). There is an item that increase max skill slot by 1. But its one time usage only on pet. i.e. pet can hold 5+1 skills max.

—>Trait can be changed, by an item from Skydweller lottery again (Note lvl 120+ Elysium Proclaimation only, before that its a diff lottery). From the pic above, Mini attack max for magmast pet is 5735, he had changed trait to mini attack and pushed to its 6.6k which is the absolute max for magmas mini attack. Change the trait to something else will not cut it back to 5735. Usually with trait, you can expect to push about 1000 more growth in addition to its original max.

—>Pet speciality can be changed, by an item that comes from a vanguard quest reward (An NPC look like Veteren War Master, Such npc is not available here yet). Main-Speciality increases affecting skills by 30 lvls, Sub-speciality increases it by 10. Each skill has a affectious speciality.

—>To make lvl 91 pet skills. Without counting pet speciality effect, u need 30 skill jades from Pet Tamer Totto, and the rest 60 lvls comes from the 50 gold lottery item from skydweller skill jade with star >.>. So that’s 90 lvls. Pet speciality will save you some jades, but changing them will change the effected skill lvl. Won’t stay like trait to growth. I.e if your pet has a lvl 30 skill because of a speciality it holds, it is considered lvl 1 by skill jade. So idealy u only need 30 skill jade, and 30 star skill jade to make it 91, but change pet speciality will reduce it to lvl 61.

—>Pet’s Passive skill (not fighting spirit which u know where it comes from) Can also be changed, Using Transmigration Flag from Pet Tamer Totto. There is a list of all possible ones, but they all very similar, except some increases material gathering apptitude, some increase attack and crital rate.

—>If you are thinking about raising a pet for pk. My suggestion is don’t, unless you are super rich and Make one close to above. They hardly make any diff at grade 12 . except for some cheaky skills like stun etc. However, pet fusing and esperating will help in pk, but again u need to have high grade.

—>Pet lvling after 140 is very slow. In fact I can say its not much easier than lvling a char from 140-150. Also pet affection limit increases when you feed him celebeans (regarless of fail or sucess it always adds a bit). Retame a pet will half the max affection. Affection affects critical rate and critcal damage. Around 5000 affection will give 3% critical and 30% critical damage. Quite a bit if you count it to 50k

—>Pet from free lunch quest is not worth keeping. For one simple reason, It cann’t be trained with celebeans or skil jades. Also it has no material apptitude, ugly appearance.

Credits go to slidstni.