Forget what I said before, that was stupid. I think I've figured out the underground class!
First of all, just pointing this out in the very beginning, they get a low low low (like seriously, enough to encourage underground travel but not enough to discourage above ground travel.) poison debuff when topside.
Second, they can climb walls
Third, they can craft things like picks and shovels and torches without a table.
Forth, they can only eat things like dirt and stone.
Fifth, they can make columns of dirt or stone appear out of the ground, but this will have to be a high level ability.
Sixth, they don't suffocate inside of blocks.
Seventh, they can turn special types of blocks into glowstone.
Eighth, their pvp is always on, but anyone who attacks themses pvp also gets turned on.
Ninth, they have enhanced strength.
Tenth, speeds up when they're twenty or so layers underground by sea-level.
Eleventh, they get special buffs during the full moon that appears in the night sky.
Twelfth, they can spawn puppies with only one wolf or another diamond dog.
Thirteenth, touching water gives them a slow debuff for say thirty seconds.
Fourteenth, has advantages when attacking skeletons due to the fact that dogs love bones.
Fifteenth, can fish in dirt for silver fish with fishing pole.
Sixteenth, can time travel.
Seventeenth, lava doesn't deal as much damage as fast.
Eighteenth, as any dog owner knows, is extra vulnerable to creepers.
Nineteenth, is just a little bit taller than other players. Two and a half blocks.
Twentieth, slimes are more likely to spawn when a diamond dog is around.
Twenty first, extra fast mining ability.
Twenty second, alchemy, changes all articles of iron to gold.
Twenty third, bark, scares off passive mobs.
Twenty fourth, untimely demise, upon death drops five-ten rotten flesh and one-three bones.
Twenty fifth, a new type of chat filter, changes person to dog and the like.
Twenty sixth, diamonds are sometimes hidden from sight, but drop one-three diamonds when mined.
There we go. not another 'me want diamond dogs derp' post. I want diamond dogs, and here's how you go about doing them. good luck.