ZWarren

Rofellos got it! It's [card]Whizbang the Wonderful[/card] by the way.

"hide and seek" refers to him/Zayle's shifting to the various heroes upon starting a game.
Second line refers to them being rarely heard, requiring RNG to do so.

Okay, now …

A master at hide and seek,
Only few have even heard him speak.
Roosters, beware!
He sits on a chicken just to make you despair.

"The best bang for your buck"
Well, a bunch of extra cards might sound great,
…

Nope. The "Rooster" might not be familiar to veterans of Hearthstone.

Nope. However, the chicken in line 4 does refer to specifically an [card]Angry Chicken[/card]. 
Your guess on the first two lines is close, too.

Nope. A hint regarding the second line, it refers to him speaking in a game of Hearthstone.

A master at hide and seek,
Only few have even heard him speak.
Roosters, beware!
He sits on a chicken just to make you despair.

[card]Anka, the Buried[/card]?

Small and weak 1/1s Deathrattle (ie Skull with Bones symbol) minions?

Quote From anchorm4n

I like them small, I like them weak

Juicy bones are what I seek

I like them weak, I like them small

Listen up and hear my call!