Pet Training
This is a bit of a work in practise, not so much an "informational" page as it is a guide page. I reckon we should make a category for "guides" where users can "ramble" in "their own words".
Contents
Initial Selection
Pet training begins by getting a pet worth training. If you're going to invest time in making your creatures more powerful, it stands to reason that you should first spend some time making sure the initial specimen is as powerful as possible.
There are a few aspects of your minions that you need to take heed of. First off, set yourself up a macro to activate the Animal Lore skill (as a tamer you should already have a fair bit of this, but if not, remember that humans get an inferred minimum of 20 points in every skill). Using this on your pets will grant you a detailed window which goes through their strengths and weaknesses.
Stats
Use Animal Lore to check out your pet's stats. Your character's stats can each be trained to a maximum of 125, and this is also true for your pets. However, unlike characters, pets have no "overall" stat cap, and so Strength, Dexterity and Intelligence can all potentially reach their maximums.
Pets gain stats by exercising their skills, but gain them far faster if using skills that are already capped out. A good way to get stats up quickly (particulary Dexterity, which can be very slow to train) is to raise Resisting Spells first, and continue to have spells inflicted on your pet after that has reached it's maximum.
Each time a creature gains in a stat, it will also gain in a related statistic - For example, improving Strength will also improve Hit Points. Therefore you want to tame a pet with the highest hit point total possible (so as to have the best starting point to train from), and the lowest strength value possible (so as to have the greatest number of potential gains to the hit point total).
An exception to this rule is when a creature's initial strength value can start at 125 or higher. As you won't be able to improve it any further, you therefore wish to tame a creature with both the highest strength and hit point totals possible. Similar rules apply to Dexterity (which improves Stamina) and Intelligence (which improves Mana).
Strength
Every time a creature gains a point of Strength it's maximum Hit Points will also increase by one. This also determines your pet's base damage output.
Dexterity
Every time a creature gains a point of Dexterity it's maximum Stamina level will also increase by one. The more stamina a pet has the faster it can strike and the faster it can travel.
Intelligence
Every time a creature gains a point of Intelligence it's maximum Mana level will also increase by one.
Resistances
Use Animal Lore to check out your pet's resistances. Unlike your character (which starts with no resistances and gains them by donning armor), pets come with their own "base" resistances which vary between specimens.
Now, your character can improve his or her resists by training Resisting Spells (albeit by a very small amount) - While your pets can also train in this skill, they will not receive a resistance bonus from it. Hence they cannot improve their initial resistance levels at all - make sure the pet you choose to train starts out with the highest levels possible for that particular creature type (especially in terms of physical resistance).
Skills
Any skill a creature has can be trained by use in the same way as your character's. They cap out at 100 but are subject to no "overall" limit, so if a pet has a skill it can max it out (regardless of how many points other skills might be using).
Because of this it's easiest not to worry too much about how high the starting skill levels are. The exception to this is when a pet's skill can start out at over 100 - because you can't train to those higher levels, you'll need to tame creatures which have those particular skills as high as possible.
Training Resisting Spells
Resisting Spells might not seem like the most useful skill, but it's likely the best place to start. The reason for this is that pets can stats faster by working skills that have already reached their maximum, and RS can be maxed out very, very quickly.
Travel to Old Haven with your pet(s) and a good supply of bandages. Wander around the area telling your pets to "kill" and Spectral Spellbinders you encounter. As soon as you can, tell them to "follow" you again and the 'Binder will follow your pet blasting it with debuffs.
If you cannot find any Spellbinders, simply kill all the undead spawn in the area and a fresh wave will appear in short order. If you have a particularly weak pet you'll need to keep a close eye on it's health bar, a full Lower Reagent Cost suit helps maintain a lengthy session of Greater Heal spells (but be careful to conserve your mana as much as possible).
Once you've got enough Spellbinders (I typically "collect" them until my pet is surrounded and can no longer move, this usually happens at about a dozen) simply sit back and let them cast away. While their spells do no damage at all, the eight closest to your pet will be able to deal melee attacks.
Once Resisting Spells has hit it's maximum, continue to sit and heal your pet until Dexterity has done the same. Dex can be arduous to train in more practical situations, so getting it over and done with now (while the gains are relatively fast) will save you plenty of head aches later.
A little known bonus that comes with dex is that it not only improves a pet's strike rate, but it also improves it's movement speed. At it's standard maximum of 125 your pets will move at a respectable pace, keeping up with you when unmounted or when slowly riding.
Creatures which spawn with dexterity levels higher then 125 (such as the Rune Beetle) move at very fast speeds. This is why Paragons can be especially difficult to outrun at times.