Dog combat training

Lots of animals a party might want to bring on their adventures (from donkeys to goldfish) have pretty low stats and will be hard pressed to survive. This is understandable since most animals arent really fit for combat. But it hurts Dogs particularly. Mastiffs have an average of 5 hp and a max if rolled of 9. Thats far below even the standard wolf.
Given all this I thought itd be nice if there was even a small way to extend the time dogs can be useful without needing to create a custom stat block or homebrew a players class.

The simplest way I can think of is combat training:
Each level of training requires an experienced trainer, a certain amount of downtime for the animal to be trained, and a gold fee. Each level of training can only be completed once.
Level 1: Takes 2 weeks of training, and costs 200gp. The animal adds 1 of its hit die + its con modifier to its maximum hp
Level 2: Requires level 1 training. Requires 2 weeks of training, and costs 300gp. The animal gains pack tactics, and gains 1 hit die plus its con modifier to its max hp.
Level 3: Requires level 2 training. Requires 4 weeks of training, and costs 400gp. The animal increases its strength by 1, its bite damage increases to 2d6 + its strength modifier, and it gains 1 hit die + its con modifier to its max hp.

This lets your players increase their dogs survivability without significantly altering game balance. Training a proficiency for your player takes much longer than a couple weeks so you can extend it if needed, I picked these durations to fit more closely with what kinds of downtime most parties are willing to take. Adjust as you see fit. The result of level 1 training is essentially a slightly weaker wolf, for level 2 its a slightly stronger wolf, and for level 3 its basically a notably weaker dire wolf.  The average hp of the final version is about 15 (with a max of 27), its not extraordinary but I think combined with barding it makes a guard dog for a druid or wizard much more possible for low level adventurers.

Or buy a few dozen goats for 1 gp each.

You should be able to adapt the Sidekick rules in Tasha's to work for a dog.

The progression makes sense, especially because it gives the party a reason to invest downtime and gold instead of simply handing the dog a stronger stat block. I’d probably make the trainer requirement matter mechanically too, maybe a failed training check means the downtime is lost, while higher-level trainers are harder to find.

Another small consideration is equipment. A trained dog with decent protection becomes much more viable without needing extra damage or abilities. That also fits the real-world role of practical dog gear, which is a focus of Supreme Dog Garage, particularly collars, harnesses, and walking equipment.

Overall, Level 1 and Level 2 feel pretty conservative. Level 3 is where I’d watch the balance, mainly because Pack Tactics plus increased damage can make the dog surprisingly effective when fighting alongside several party members. The idea itself is solid and feels more rewarding than simply replacing the dog with a wolf.