I have a player wanting to purchase a "war trained" mastiff as a mount (think warhorse) and he is getting this from the MM description:
Mastiffs are impressive hounds prized by humanoids for their loyalty and keen senses. Mastiffs can be trained as guard dogs, hunting dogs, and war dogs. Halflings and other Small humanoids ride them as mounts.
I looked but can't find any description of the benefits or differences of a war dog. Any idea where I can find them?
The difference here isn't in the mechanics of the stat block for a mount, but in the way you interact with your mount in battle.
The Players Handbook (198) covers the rules on Controlling a Mount:
You can control a mount only if it has been trained to accept a rider.
The initiative of a controlled mount changes to match yours when you mount it. It moves as you direct it, and it has only three action options: Dash, Disengage, and Dodge. A controlled mount can move and act even on the turn that you mount it.
An independent mount retains its place in the initiative order. Bearing a rider puts no restrictions on the actions the mount can take, and it moves and acts as it wishes. It might flee from combat, rush to attack and devour a badly injured foe, or otherwise act against your wishes.
An untrained mount will always act independently and not listen to commands or direction by the rider. It is effectively controlled by the DM and will respond naturally to the environment around it.
The real difference here is that a controlled mount can act under your orders (limited, but options decided by you), while an untrained mount is akin to temporarily riding a wild beast and it will do things on it's own accord that may not be what you want.
It's about when and how the mount responds - not a difference in what the animal can physically do.
The differences between a riding horse and a warhorse, as listed in the PHB, are that the warhorse comparatively has +2 Str, +2 Dex, +1 Con, and +1 Wis, has an extra hit die, does a bit more damage, and can attack to knock prone. It can follow commands, and may be proficient in wearing barding. It costs 400gp instead of 75gp.
If you apply equivalent changes to the mastiff as written, you wind up with a stat block that is very similar to that given for the wolf. For a player that wants a "trained riding war dog", I would simply use the stats of a wolf, allow it to be a controlled mount and follow commands and wear barding, call it a trained war mastiff, and charge the character about 150gp for it.
The DM will have to decide how rare this is, with regard to people's reactions to it, as well as to the cost and availability of barding, military saddle, etc.
