You have been called man's best friend since you're kind was first domesticated. Now its time to prove it, against giants, dragons, and even the undead. ..... The things you do for love!
A Dog's Life: You are a dog. Your race and background instead indicate the race and background of your original You gain all the normal benefits of that race and background, except that you are a Beast rather than whatever type of creature you would normally be, you age in dog years, you cannot speak or write in any of the languages you know, and your base Speed is always 30 feet. You don't have thumbs (or hands, for that matter) and cannot wield weapons, but your unarmed attacks do 1d8 damage, goes up by 1d8 per 5 levels (5d8 upon reaching dog level 20). Likewise, you cannot use tools, but you give allies adjacent to you advantage on any tool use check for which you are proficient. It is generally not possible to multiclass into Dog without magical assistance, but multiclassing into something else uses the normal multi-classing rules with a requirement of 13 Int from the Dog portion of the requirements. While nor wearing armour, your AC counts as being 12 + Dex Modifier + Con Mondifier.
Man's Best Friend: If a Humanoid or Giant pets you, plays fetch with you or gives you food, you become Best Friends until you or they take a long rest. They understand what you are trying to communicate with your woofs, wags and growls (treat this as a common language), and you may spend your bonus action to move up to 30' directly toward one of your Best Friends. You may have an unlimited number of Best Friends - Dogs aren't the best at counting.
Keen Senses: You gain advantage Wisdom (Perception) checks that rely on hearing or smell. If you have an item from a person you are trying to sniff out, add your proficiency bonus to any Wisdom Survival checks to track that person. This stacks with the regular proficiency bonus for being proficient in Survival.
Catch: At 2nd level, you learn to play catch! As a reaction, you may move up to half you speed in order to intercept a thrown object which could fit into your mouth. You must put yourself somewhere along the line between where the object was thrown and where it would have landed (or hit). If this object was thrown as part of an attack, the attack does no damage to anyone if you catch it. Once you've caught an object, your mouth is full and you may not use your bite attack until you release the item you've caught. You may only release an item to catch another item, during a short rest, or when someone proficient in Animal Handling or Sleight of Hand spends an action to get the item away from you. You also drop items held in your mouth if you become incapacitated.
Growl: At 2nd level, you also learn to growl very loudly. As an action, you may force one creature within 50' who is NOT your Best Friend to make a Wisdom Save or become Frightened with a DC of 8 + Proficiency Bonus + Charisma Bonus. This lasts for ten minutes, or until they become your Best Friend. If a creature saves against your growling, you cannot (usefully) growl at that creature until after the end of your next turn. (For example, if you growl at a dragon on turn 1 and it saves, you cannot usefully growl at it until your action on turn 3. But you could growl at the wererat in the corner instead.) If you growl at a creature that is immune to being Frightened, all of your Best Friends gain advantage on attack rolls against that creature until the start of your next turn.
Very Good Nose: At 6th level, you may reproduce the effects of Detect Evil and Good at will. At 9th level, you may instead use Detect Poison and Disease. At 11th level, you may use Detect Traps. At 15th level, you may use Sense Emotion. At 17th level, you may Detect Magic. Unfortunately, you can only detect things within a foot radius of your nose using these spells.
Doggy Archtype: At 3rd level, you determine learn who the Good Dog is and whether or not it is you.
Uncanny Dog: At 5th level, you gain the ability to communicate with all humanoids, even if they are not your Best Friend (yet). Also, whenever an attacker that you can see hits you with an attack, you can use your reaction to halve the damage against you.
Well Lore: At 5th level, you have an instinctive knowledge of the direction, distance and condition of the nearest creature trapped within a well. While Good Dogs use this to mount rescues, Bad Dogs use this to create compasses.
Play Dead: At 12th level, you learn to play dead! Whenever you would normally make a death saving throw, you may instead choose to recover a hit die as if you had just taken a short rest. Additionally, whenever you are wearing a collar and would be disintegrated, imploded, incinerated or otherwise instantly killed, you vanish and leave the collar behind. If even a single creature mourns your loss, you reappear during their next long rest alive and whole. These abilities can only be used once per long rest each.
Go to Heaven: All dogs go to Heaven, regardless of alignment. Most must wait until they die. But at 20th level, you don't have to. You may Plane Shift to any Good-aligned Outer Plane (or campaign equivalent) as a bonus action. You can also Plane Shift back to the last non-Heaven location you were in in the same way. Both of these abilities require a short rest in between uses. If you're a Good Dog, the Celestials will even be pleased to see you.
A collar with your name on it, and (a) water bowl or coating of mud; (b) ball, stick or bone; (c) an unlimited supply of love, slobber or shed fur (pick two).
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Good Dog
A Good Dog: At 3rd level, any creature who inquires aloud as to the identity of the Good Dog while within earshot of you instantly realizes that you are the Good Dog. All Celestials and children under the age of 10 are automatically your Best Friend.
Belly Rubs: At 3rd level, you may attempt to solicit food and belly rubs from Humanoids and Giants by wagging your tail at them. Such a brazen display may only be done once per short rest and requires that you remain prone while soliciting affection. All such creatures within 50' of you who fail a Charisma saving throw (DC of 8 + Proficiency Bonus + Charisma Bonus) are charmed by your antics until such time as they become your Best Friend, or you attack them, or one minute has passed.
Face Licks: At 8th level, you may lick adjacent Best Friends as a bonus action in order to let them make an additional saving throw against any effect(s) which would leave them charmed, frightened, incapacitated, diseased, poisoned or otherwise unwell. If they are dying, they automatically stabilize. This ability may only be used once per long rest per person.
The Goodest Dog: At 14th level, you may call upon the Goodest Dog once per long rest. Treat this as using Conjure Celestial to conjure a Couatl, except that the Couatl's true form (for its Change Shape ability) is that of a very friendly dog.
Cold Noses: At 18th level, your Face Licks ability works once per short rest per person instead. Once per long rest, it can restore one of your Best Friends to full health as long as they haven't been dead for more than a minute.
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Bad Dog
A Bad Dog: At 3rd level, your Best Friends become "Best Friends" instead. You may not have regular Best Friends, only "Best Friends". The first time that you bite someone who is your "Best Friend" in round, it deals an extra 1d6 damage per two Dog Levels you have. Being "Best Friends" doesn't stop you from growling at someone, either. Undead and anyone with infested with fleas is automatically your "Best Friend".
Good Dog?: At 3th level, you are filled with resentment at a world that does not think that you are a Good Dog. You gain proficiency with the Disguise Kit and with Deception, and apply your proficiency bonus twice when attempting to conceal the fact that you are Not A Good Dog. While you still may not use an actual Disguise Kit (no thumbs), you can roll around in mud to the same effect.
Fleas: At 8rd level, your fleas get entirely out of control. Any creature not immune to poison who touches you, attacks you or becomes frightened of you is automatically infested with the horrible little things until they take a bath. With soap. Once cured of your fleas, they are immune to your particular strain for a week.
Mail Bane: At 14th level, you gain a special hatred for all who would flaunt their filthy penmanship by carrying the written word around with them. All creatures who are carrying letters, books or other forms of writing becomes your "Best Friend" - you are able to recognize such creatures on sight. Additionally, whenever you eat a spell scroll, you gain the ability to cast the spell within once (as if by reading the scroll) regardless of whether you meet the normal requirements to do so.
Dire Heartworms: At 18th level, your fleas somehow become infested with Dire Heartworms. Once per long rest, when a creature dies when infested with your fleas, you may have two dozen dire heartworms (mechanically identical to constrictor snakes; see the Monster Manual for details) erupt from the corpse. These creatures are frightened by you and your companions. Roll initiative for the heartworms as a group - they each attempt to constrict, kill and crawl into a nearby creature other than you and your companions, with any heartworm unable to find prey slithering off in search of warmth and food. These heartworms (thankfully) expire after spending one hour (cumulatively) outside of a warm body.