Sonic Adventure New Year's DLC (found "Sonic the Hedgehog" holiday-themed DLC; 1998)
According to Sonic Retro, this was the icon displayed for the DLC in the Dreamcast's VMU manager. However, it may be incorrect as this is also the Samba GP icon.
Status: Lost
The Sonic Adventure New Year's DLC is a piece of downloadable content released by SEGA to commemorate New Year's 1998. According to Sonic Retro, when active, the DLC would place a traditional kadomatsu, a Japanese decoration said to welcome ancestral spirits on New Year's Eve. It was initially released on December 26th, 1998, only to be patched for a bug later, on December 29th. It was removed on January 7th, 1999. Apparently, it was 66 blocks.
A Japanese Geocities site claims to have it for download, but the link is broken. No other save file archives around the internet seem to have any trace of either the original or fixed version.