Well, the why is to commemorate the release of Dragon Quest Heroes, the new PS4 and PS3 Hyrule Warriors-like crossover that will combine the world of Square Enix's RPG series with action crafted by Dynasty Warriors devs. That's due in spring 2015. As for what that thing is - the happy little slimy dude - it's a removable USB cover rather than a permanent fixture. We're filing it under S for "Sure..." and Y for "Why not, I guess?"
There's no news on when the console will be released or at what price, let alone whether we can expect the game and/or ooze to slime its way over here.
The Metal Slime PS4 wasn't the only new thing Sony showed off yesterday, because that would be super weird. For the lowdown on all the major announcements from yesterday's presser, check out our round-up of the highlights.