PSO2 continues its tradition of having a Sonic event each year around his birthday. This time, the lobby is more elaboratly Sonic-styled. With more Green Hill palm trees AND the edition of rings (that you can really collect!) along with fun star bumpers to bounce you around.
As you can see, all the same giant statues appear (Already pictured here on gear) with the big Sonic in the center. More fun was in store in 2015 for Sonic fans, as the fancy lobby had a secret.

Rings were scattered about the lobby, and you could pretend to be Sonic, running around collecting them all. However...if you collected/found all of them, the big statue in the center would change: a chaos emerald would appear in his hand (seen at left) along with a chime noise.
Jumping your character onto the star bumpers has the same effect as in a Sonic game: flinging the character far away (and along a row of rings) All of the items make authentic sound effects.
It's awesome that the Phantasy Star game continues its ties with Sonic themed fun for everyone.

And what's that odd creature?
It is Nyau, a mysterious cat-like...person...thing. It started out in old Phantasy Star games as more of a cat. In PSO2, it's updated into a little mascot-type person that "travels between dimensions" to "challenge the Arks (players) to a fight". When he loses, he cries & dissappears, generally allowing a large (and actually evil) boss monster to appear in his place.
The character is generally dressed up for seasons/occaisions in the game, and then let out to appear. So such things as "spring/sakura" & ceremony dress ones can show up.

In this case, they've dressed him...sort of...Sonic like. It's a bit of a combo between his natural color (that beige) & looks like a spiked hood & dyed fur. The weapon he has is meant to look like a board from Free Riders. There is a chance the item can drop from the fight for you, too.