Hearthpwn

 The Ancient One

Okay, so here's my take. I'm thinking it's Mark of Y'Shaarj

Speculated I may have been (41% of people thought it would be very good, 33% thought it would be playable)

I’ve been marked, now you will too (the card has Mark in its name, and draws a card)

No longer will they be so lean (makes a small minion bigger)

Centuries of old, I will undo (Referring to Y'shaarj himself)

Alas, I was never seen (The card didn't end up seeing a ton of play)

For my numbers were too few (Something about the card not being enough to justify playing)

In obscurity, I now dream (I kinda forgot about the card...)

As I’ve returned to where the Kraken’s wilds grew (referring to it being a druid card that affects beasts from the year of the kraken)

Not quite. Good guess though.

my guess would be Y'Shaarj, Rage Unbound himself.

"speculated I may have been" I don't know what people thought about y'shaarj before he was released, so I am just guessing here.

"I've been marked, now you will too" this hints to Mark of Y'Shaarj

"no longer will they be so lean" again, this hints to Mark of Y'Shaarj

"centuries of old, I will undo" because he is a old god

"alas, I was never seen"  I was new during the old gods meta, but from what I now, y'shaarj did not see much play.

"for my numbers were so few" I guess not a lot of people owned him?

"in obscurity, I now dream"  because he was not played a lot

"as I've returned to where krakens wild grew" after year of the kraken went to wild, y'shaarj saw more play in big priest, and a little bit in y'shaarj hunter.

not the best guess, but I have nothing else, that "I've been marked, now you will too" just has me stumped.

Also close!

Here's a second clue. A little shorter this time

You thought me as Mark

But I may have been Chris

I’m whispering ‘La Bamba’ in the dark

But my claws still do not miss

Addled Grizzly

Ancient Harbinger, I think. Unlike the other guesses, I fail to recall anyone playing this. 

EDIT: Just saw the second clue. Yeah, I guess Grizzly's more likely. 

Winner!

Here's the clue rundown.

Speculated I may have been - > Nearly 50% of hearthpwn users thought it was "playable", another 25% thought it was "very good"

I’ve been marked, now you will too - > referencing his visible marks from Y'shaarg, but also references Mark of the Lotus, as the buff is identical.

No longer will they be so lean - > again referencing him buffing cards

Centuries of old, I will undo - > hint to the set, Whispers of the Old Gods

Alas, I was never seen - > Turns out card is bad

For my numbers were too few - > 3 cost 2/2 isn't good enough for aggro

In obscurity, I now dream - > a second hint to the set

As I’ve returned to where the Kraken’s wilds grew - > referencing the wilds, as it is a druid card, as well as the Year of the Kraken.

Clue 2:

You thought me as Mark - > Y'all thought it was Mark of Y'shaarg

But I may have been Chris - > Referencing the flavor text of Mark of Y'shaarg, because the art is super similar.

I’m whispering ‘La Bamba’ in the dark - > Probably the biggest hint, I couldn't help but quote some of my favorite flavor text that Blizzard has done.

But my claws still do not miss - > It's an animal with claws. 

Your turn Scarifar!

Sliding around
At the speed of sound,

Use him in OTK's with a mage,
but maybe not with unbounded rage.

He's no Dr. Boom or Lich King,
but in our eyes, he is just as meta-defining.

So my first thought was Faceless Manipulator

The only thing it fails to complete would be “sliding around at the speed of sound” but I can’t think of anything that does that.

Use him in Otks with a mage -> Faceless + Malygos combos, in WOW lore malygos is a mage turned dragon.

But not with unbound rage -> he’s horrible with Y’ashaarg, rage unbound.

and he copies both boom and the lich king.

quite confident this isn’t right, but I’m just putting thoughts down 

Quote from Datrevdo >>

So my first thought was Faceless Manipulator

The only thing it fails to complete would be “sliding around at the speed of sound” but I can’t think of anything that does that.

Use him in Otks with a mage -> Faceless + Malygos combos, in WOW lore malygos is a mage turned dragon.

But not with unbound rage -> he’s horrible with Y’ashaarg, rage unbound.

and he copies both boom and the lich king.

quite confident this isn’t right, but I’m just putting thoughts down 

 I like your train of thought, but sadly it's not even close.

The best guess I can think of is Banana Buffoon

Sliding around = Slipping on banana peels?

Speed of Sound/Mage OTK = as far as I'm aware it was a huge help to Exodia Mage since it gave you two 1-mana spells

Bad with YShaarj obviously

Not sure how it makes sense for the last two lines though.  I do have one more idea if this is wrong that might make more sense if I can just find a Mage OTK deck that uses it, but it'd have to be a really weird and niche deck.

EDIT: Ok nevermind I changing this to Majordomo Executus

Sliding Around = He's a snake thing

Speed of Sound = TOO SOON

OTK with Mage = There was/is an OTK deck with him and Coldarra Drake for maximum overkill

Bad with Ysaarj = If you pulled him out with Ysaarj you'd probably be killed on your opponents turn

No Dr Boom or Lich King = like those two he was a "big boss" of an expansion kind of, but not good

But in our eyes... = From what I recall, he was pretty well loved by the community

Quote from Pyrotrap >>

The best guess I can think of is Banana Buffoon

Sliding around = Slipping on banana peels?

Speed of Sound/Mage OTK = as far as I'm aware it was a huge help to Exodia Mage since it gave you two 1-mana spells

Bad with YShaarj obviously

Not sure how it makes sense for the last two lines though.  I do have one more idea if this is wrong that might make more sense if I can just find a Mage OTK deck that uses it, but it'd have to be a really weird and niche deck.

EDIT: Ok nevermind I changing this to Majordomo Executus

Sliding Around = He's a snake thing

Speed of Sound = TOO SOON

OTK with Mage = There was/is an OTK deck with him and Coldarra Drake for maximum overkill

Bad with Ysaarj = If you pulled him out with Ysaarj you'd probably be killed on your opponents turn

No Dr Boom or Lich King = like those two he was a "big boss" of an expansion kind of, but not good

But in our eyes... = From what I recall, he was pretty well loved by the community

 That does fit quite well, but again, no.

Maybe I should provide some more clues: 

On ice, he slides around
At the speed of sound,

You can use him in OTK's with a forgetful mage,
but maybe not with unbounded rage.

He's no Dr. Boom or Lich King,
but in our eyes, he is just as meta-defining.

He may join Rexxar in his eternal hunt,
but maybe not, as he's quite the runt.

Oh but of course!

Snowflipper Penguin!

Not really sure what kind of OTK deck it was in, but everything else adds up

Quote from Elvaeyn >>

Oh but of course!

Snowflipper Penguin!

Not really sure what kind of OTK deck it was in, but everything else adds up

 Eyyy there we go.

Snowflipper Penguin, or pretty much any 0 cost minion, could be used with Ancient Mage and other battlecry minions that give spell damage (especially in Shaman with Shudderwock) and just blast away with some spells.

Anyway, your turn.

Alright, I've been sitting on this one for a bit:

So you feel empty?
I can fill you up inside
Won't you have a taste?

Might be too easy but we'll see

Oh darn Penguin was the first guy I thought of for the sliding part, but I tried to search for OTK mage decks with him, but couldn't find any.

Quote from Elvaeyn >>

Alright, I've been sitting on this one for a bit:

So you feel empty?
I can fill you up inside
Won't you have a taste?

Might be too easy but we'll see

 Refreshment Vendor ?

Quote from Elvaeyn >>

Alright, I've been sitting on this one for a bit:

So you feel empty?
I can fill you up inside
Won't you have a taste?

Might be too easy but we'll see

The first thing I thought of was also Refreshment Vendor, but I guess this works too. 

Priest of the Feast ?