Way of the Dog (Monk Sublcass)
Certain monastaries in the forgotten parts of the world train in an obscure form of martial arts. Each monk is gifted a puppy of a highly specialized dog breed and the two are raised together. The monk takes care of the pup and trains it himself, forging a lifelong friendship. As they grow up together, the dog is infused with the same ki that its master learns to harness, binding their souls together as they learn to fight in total synchronicity, using one another as an extension of their respective bodies. The result of years of ardurous training is a formidable team and an unshakable bond. Foes are treated to a flurry of combined strikes, bites, and grapples as monk and dog work in perfect harmony to take down any enemy. The core of this Monastic Tradition is the bond between man and beast; with each journeying to greater heights of power and skill, all while learning from one another. As one attains greater strength, so does the other in turn, encouraging one another's path to trancendence...a goal they shall reach together.
Starting at 3rd level, your dog companion is not only your best friend, but an integral component of your martial arts. Your dog is always either the same size as you or one size smaller (up to the player but the dog needs to be able to perch on your character's shoulders or fit between your legs) Utilizing the precise training you and your dog have undergone, your dog occupies the same space as you in combat, positioned either between your legs, on your back, or balanced on your shoulders. Once per turn, you can replace one of your unarmed attacks with one of your dog's attacks. Your dog's attack damage utilizes your martial arts die scaling but the die is rolled twice. Your dog's attacks have the Reach trait and inflicts bleed for 2d4 damage for 1 round, representing advanced techniques where your dog might leap off your shoulders or dart out from between your legs to harry a creature.
Starting at 3rd level, your years of training together in the martial applications of ki have bound you and your dog emotionally but it was the tradition of Soul Merging, your monastery's esoteric tradition, that forged yourselves into a single soul, binding your destinies forever. Your dog shares your Hit Points and Armor Class, any attacks or damage directed at it are reflected back to you. Additionally, any later Class Features that directly affect your physiology (i.e. Timeless Body or Empty Body etc.) apply to your dog as well, representing your shared seat at the table of true power and enlightenment.
"Who's A Good Boy?"
Let's face it, your dog is absolutely adorable and everyone knows it. Charisma skill checks (Persuasion) you make have advantage when your dog is within 5ft of you.
Flurry of Fangs:
Starting at 5th level, when you spend a bonus action and 1 ki points to perform Flurry of Blows, you can replace both unarmed attacks with two of your dog's attacks, inflicting bleed for 1d6 damage per turn for 1d4 turns on the target (unless the target is immune). As referenced in Inudo, these attacks have the Reach trait.
Starting at 6th level, further training and bonding with your dog imparts increased levels of ki into the beast, channeling greater power and accuracy into its bite. Your dog's attacks count as magical for the purpose of overcoming resistance and immunity to nonmagical attacks and damage as well as gaining an additional +4 to hit (you do not gain this attack bonus if you have disadvantage on the attack roll).
Starting at 7th level, instead of using your dog's attack, you may choose to have your dog grapple a creature instead. Regardless of your dog's size, it is always able to grapple Medium and Large sized creatures (A Gnome Monk's tiny dog is able to achieve this through clever manipulation of ki that magically enlarges the dog's jaws to fit its targeted creature.) This works as a traditional grapple (Athletics vs Athletics). While the target is grappled, they receive 3d4 piercing damage per turn, representing the dog's fangs buried in their arm. Any damage dealt to the dog while it is grappling a creature is reflected onto the monk, representing their joined souls. The grapple is broken if you move more than 20ft away from the targeted creature as this prompts your dog to return to your occupied space. If the grapple is broken by the targeted creature, the dog returns to your occupied space. Your dog is vulnerable to attacks of opportunity (and you by proxy) when returning to your occupied space.
At higher levels, the number of damage dice increases by 1 at 11th level 4d4), 15th level (5d4), and 17th level (6d4).
Starting at 13th level, as a bonus action, your dog perches itself atop your shoulders and projects some of your shared ki into a terrifying growl. The target must make a Wisdom saving throw (DC=8+WIS+PROF) or be Frightened for 3 turns. The target creature may attempt to shrug off this status effect at the end of their turn each round by making a Wisdom saving throw. This feature may be used three times before requiring a long rest to replenish it. At higher levels, the duration of the Frightened status increases by one turn at 15th (4 turns), 17th (5 turns), and 19th level (6 turns).
Starting at 17th level, you can spend 5 ki points to have your dog channel its ki to create an astral projection of itself. The astral projection has 5 hit points and its AC is half of your player level + proficiency. The astral projection has its own action and uses the same attack and damage rolls as your dog but the damage type is Force and does not inflict bleed. While the astral projection is present any attack rolls directed at the player or dog have disadvantage and melee weapon attack rolls made towards a creature have advantage. The astral projection lasts for 3 rounds unless it is destroyed, you become incapacitated or die, or you dispel it early. This feature has 3 uses and requires a long rest to regain all expended uses.