UPDATE: It now appears that Raid Eggs are back for ALL RAIDS.
A feature dropped by Pokemon Go has made its triumphant return...at least temporarily. Players in San Francisco are reporting that a Legendary Raid Egg has appeared over the gym at the Bay Bridge, complete with a timer counting down to when the egg will hatch. While unconfirmed at this time, it's speculated that the Bay Bridge could be the site of a test EX Raid, which would make sense since Niantic is based out of San Francisco. [Editor's Note: The Legendary Egg was actually a Raikou.]
Originally, Raid Eggs would appear two hours before the start of a Raid to alert players when the Raid would take place. The idea was that a Raid Egg would give players time to tell their friends about the Raid and gather them at the Raid's location when it would hatch. However, the eggs disappeared from the game without warning after Legendary Pokemon were added to Pokemon Go. While players have occasionally seen Raid Eggs randomly appear in place of Raid Bosses above gyms, this is just a temporary glitch that disappears after a few moments.
The San Francisco Raid Egg is technically the first "official" Legendary Raid Egg to appear in Pokemon Go. While image assets for the Legendary Egg (which looks metallic and has stripes) have been in Pokemon Go since July, they've sat unused since Pokemon Go unceremoniously stopped using the Raid Egg mechanic all together.
Right now, it's unclear if Pokemon Go plans to bring the Raid Eggs back just for the EX Raids, or if they'll make a wider return in the coming days. Pokemon Go is currently testing out EX Raids, so it's also possible that they're just trying the Raid Egg out to see how early beta testers like it.
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