Chester the Squirrel is Al's faithful companion for the first few Minecraft episodes. Al puts the "squirrel" on his head and walks around with him, until he dies. Al then finds another "Chester" and puts him on his head again. In reality Chester is a rabbit from the Mo' Creatures mod made by someone using the alias DrZhark. For more information on the mod the minecraft wiki link is here.
Alchestbreach sings a song for Chester in various episodes of Minecraft: Go West. He also sings a similar song when he starts playing The Elder Scrolls: Skyrim, though the only comparison is that they have the same tone. Every time he sings the song he changes it, the first version of the song goes like this:
"Chester the Squirrel in our highway adventures through the world. Chester the Squirrel oh look the ocean we're totally fucked, God damn it."
The later versions of the song are long though.