Diablo 3, unknown creature
Yes, it is a Treasure Goblin. More specifically, it is a Rainbow Treasure Goblin. These fine creatures spawn entrances to Whimsydale (which is different to Whimsyshire, at least in name).
Yep, it's definitely a treasure goblin. You got lucky, though, and it opened a portal to Whimsyshire. It's a "secret" area (but not really so secret), where everything is brighter and more cheerful, and you fight things like horses, unicorns, flowers, and teddy bears. It's meant to be a dramatic departure from the rest of Diablo 3.