It's look like each habitat in Pokemon's GO Fest is going to have its own shiny Pokemon attached to it. Heatmor for Fire, Qwilfish for Water, and now Tangela has been confirmed for the Grass Habitat! While you might be a bit confused with what's going on because Pokemon GO Fest 2020 shouldn't be out yet, it has gone live in certain parts of the world where it is already July 25th. Don't worry though, you'll get your chance at all of these increased Pokemon spawns, and to get your chance at a whole bunch of new shiny Pokemon!
Tangela has been found as a shiny (Credit to this Tweet):
As I've also mentioned, you will also be able to find Heatmor who will be available during the Fire Habitat rotation:
And there is the shiny Qwilfish who can be found during the Water Habitat (Credit to this Tweet):
There will also be a Battle and Friendship Habitat available where we can hopefully expect some other brand new shiny Pokemon to spawn! We'll keep you posted when those go live and they have been found.