For more information on collecting these cards and their Booster Fun treatments, read our guide to Collecting Tarkir: Dragonstorm
Cobra Jar is just a jar full of cobras, and when you summon it you also summon a bunch of snakes
Like I said yesterday, you'll want to keep Mareep on the bench until you evolve it into Flaaffy, once that happens I would still recommend keeping this Pokemon on the bench because it doesn't have a very high hp and neither of its moves can do much damage, at least for the amount of energy that it requires
Now that the set has launched, however, demand for this card has absolutely skyrocketed
2017-06-27 If the card copied by the token had any "when [this permanent] enters the battlefield" abilities, then the token also has those abilities and will trigger them when it's created
Making a Magic: The Gathering card is a long process