I am running a game for my son and his friends. My son bought a mastiff. I am wondering if the dog should gain experience and advance in levels. I have not thought of how to do this. If the dog does not gain levels it will eventually become useless as the party becomes higher level and is fighting more powerful monsters.
If your son is not a ranger, no.
Animals, wild or domestic, have their own statblock as described in the MM or the PHB.
However you still can homebrew this:
During the downtime between two adventures, you can suggest your son train his dog, it can gain like this (only one a time):
- One hit die
- Adding your proficiency to one skill like perception, survival
But remember it should not be too overpowered (not be as powerful as a ranger's beast) and remind your son that resurrecting his dog might not be possible, or at least very expensive.
Yes, yes they do.
This is implicit in each monster entry that gives a range of hit dice. Some will even gain a size category once they gain enough hit dice/experience. This allows DMs to have creatures be relevant to the players across several levels. See the "Advancing Monsters" sections of the monster manuals.
However, no method has been laid out for animals to gain xp concurrently with a player group, other than familiars and companions.
It has been implied across various rule books that monsters and animals gain HD as they age, though again no aging tables were provided for anything other than pc playable races... and dragons.
A simple method would be to track the time passing and simply advance the creature an HD every year or so until it reaches max HD (physical prime). Then apply aging penalties every few years or so until it reaches the average life expectancy for said creature.
Use modern life expectancy values if you have to for creatures that correspond to real life creatures, and use shortened values for hybrid creatures (Half breeds, especially forcibly mingled ones typically don't last as long as the originating breeds... unless it was mixed with a dragon, fey, or outsider, which seems to be the exception).
If the creature becomes a real part of the party, consider adding templates to it, or even awakening it, which converts it to an npc and solves the advancement problem completely. If it was awakened by an animal druid, it still might not learn common, nor be able to speak understandably, preserving the "animalness" value, if that is important.
Keep in mind that the party doesn't have to know it's awakened, could be a fun reveal or plot twist. I can just envision it now... the party has been captured, all hope is lost, when suddenly they are rescued... by the horse?!?