I and a group of friends are starting a Pathfinder campaign, and I plan to play a halfling dog rider just for the fun of it. There is one big problem : I don't plan to play any class with an animal companion or familiar class feature, and not a spellcaster in general, and riding dogs only have 2 HD and 13 HP. Then I'm pretty worried about my mount dying too easily and being too vulnerable to spell effects at high levels. Then I want to know if it is possible for an unaltered animal (one that hasn't been magically altered like familiars and animal companions) to get battle experience and grow in power like PCs do, and what can be upgraded, along HP, BAB and saves.
NPCs improve only if the GM says so
By default, according to Character Advancement,
As player characters overcome challenges, they gain experience points. As these points accumulate, PCs advance in level and power.
Emphasis mine. This means, unless the GM says the NPC does, an NPC (even an animal) neither gains experience points nor advances in levels.
Thus, if you don't want to take a feat like Leadership and have the mount as a cohort, then you've at least three choices.
- Beg the GM to implement a house rule that lets the mount gain experience points and advance levels like a PC or similar.
- Over the course of the campaign, inquire about ways to make your current mount increasingly powerful, and seize upon such ways when the GM makes them available. These will probably be house rules, too, like adding templates because of weird encounters, finding special trainers to teach the animal feats, or feeding the animal a diet of magical pet food to increase its ability scores.
- Over the course of the campaign, inquire about increasingly powerful appropriate mounts, and, when you find one, swap your old mount for a new, better one.
There are two parts of answer to consider:
- The exact wording of rulebook (read it and see, if there is a rule for it or not)
- Real world experience and homerules
I will talk about the second: In real world any animal (say from mouse upwards) can be trained to do some tricks, can take experience in fights and became better at it and can (with proper training and feeding) became stronger and more resistant to damage (at least to some degree).
Also is clearly that old street cat who fights for food and life daily is more likely to win fight with house cat, who only sits all day on window and lick milk.
So in our games we make homerules for such treating of animals, when/if some character tries to train them and we allow them to grow somehow. The same goes for all other animals too, if there is some kind of continuing training in some specific way. Maybe the core book does not agree (depends on particular gaming system), but we want to have fun and to have world, which works as we see it should, not just some schema made from paper that we would be forced to use mindlessly.