Not sure if this will help but here it goes. Different pet combinations are good for different situations:
Nightmare/Rune beetle or Rune Beetle/Bake Kitsune - probaly the best Dmg output combination. If you want to ride around places i suggest choosing the Nightmare as you cannot mount the Bake Kitsune. While this combination is great for dmg dealing it is not very good for tanking tough monsters. Its best used against monsters that will drop fairly quickly or with low dmg output, its also great to use this combination if your working with another tamer using a tank pet when doing things such as peerless.
Rune beetle 3 slots
Nightmare/Bake Kitsune 2 slots
2 Bake Kitsune - this combination may not have the dmg output of the previous combo but its still a pretty good combo which you can have a mount with. They can put out alot of dmg and have good resists to tank a little but they have low health and may not fair well against big tough monsters.
Cu Sidhe - Cu Sidhe is a good tanking pet, it has alright dmg output with a power bite and bleed attack but no magic. It has high health, pretty good resists and the ability to heal itself which makes it a good tank. It can also be mounted to make life a whole lot easier by running through dungeons etc to get to your destination. Also because the Cu Sidhe is only 4 slots you could have another mount with you and let the Cu Sidhe follow you or you could have a 1 slot pet with you to deal that extra dmg such as a Giant Ice Worm which does 90% cold dmg and can inflict deadly poison when trained. Cu sidhe is best used against monsters that take a while to kill and hit hard such as Balrons, so with your help vetting it can survive against it. Please note that only elves can tamer Cu Sidhes and they can ride them normally, in order for a human to ride one they will need to be wearing the Minor Artifact 'Pads of the Cu Sidhe'.
Greater Dragon - Greater dragons are just sheer awesomeness, they have a huge amount of hit points and great resists making them an outstanding tank. They also have a hard bite, good magic skills and devastating firebreath which gives them a very good damage output too but perhaps not as good dmg output as the first combo i mentioned. The bad thing about Greater Dragons is that they take 5 slots so you'll be on foot all the time making you vunerable. Greater dragons are best used against the toughest monsters in the game such as peerless and also against monsters with low Physical or fire resists. Greater Dragons do really well against Lady Mislende the peerless as she has low fire resist, so when at full health they do 100 dmg with their firebreath against her.
Dread Warhorse/Nightmare - I can't really say much about this combo as im yet to try it myself. But it could have potential as a dread warhorse is a super buffed nightmare, and i seem to remember a 2 nightmare combo being pretty good against dragons. Its just a shame they don't have a pack bonus
.
Dread Warhorse - 3 slots
Hiryu - Hiryu's are alright, they are good tanks, hit hard with high wrestling and are mountable. But thats about it, most people prefer to use Cu sidhes over them as they are better for PvM. But Hiryus are popular in fel PvP for their ability to dismount people.
Hiryu - 4 slots
5 1 slot pack animal combo - Now this combo is my favourite, using 5 1 slot pets with pack bonus, the best of these pets include hellcats, hellhounds, dire wolves and frenzied ostards. I have a pack of 5 hellcats myself and when i hunt with them i find it very fun, with the pack bonus their damage output is amazing but they are very bad tanks and die very quickly against tough monsters. 1 will nearly always die when i set the pack on a dragon, but have no problem with Titans or Cyclops, they eat them in literally 2-4 seconds. This combo was very popular in fel PvP a while back, and still is for a certain few tamers. The bad thing about this combo is that you cannot have a mount which leaves you vunerable. If you are working with another tamer that has lets say a Greater Dragon tanking a monster and then you have your pack of 5 pack animals attacking without any of the aggro, then the dmg dealt is devastating.
There are many different combos and pets out there you can have fun with, these are just the best/ Most popular combos. Different pets will always be better in different situations such as the strength of mobs, their weakest resist, the environment etc etc, you'll learn in time which pets are best for which situation through experience.
Hope this helped
P.S. most tamers will tame most pets for free, the only time i really see people selling pets is if they are fully trained, rarer colour Cu Sidhes, or really good Greater Dragons stat and skill wise.
