Similar to my own thoughts, Curiosity is really the engine of the deck and Carl felt like if wasnt able to get it online then he would get overwhelmed
This card, illustrated by Atsushi Furusawa, has Otter Ral scampering across a rocky area with lightning arcing off his body
Perhaps more important than that, though, is that playtest names shouldn't trip people up or confuse them
The front side, Avatar Aang, is a value engine
So if you really want to know what promos are Pauper legal, you have to find a list of all common Magic Online promos then MANUALLY LOOK UP EACH CARD to see if it has other online printings (in which case it isn't legal), unless one of those other printings is at Common (in which case it is legal)
The first choice for building a Commander deck is generally the commander, this is where your color identity comes from, and you add both partner commanders together to get the color identity