According to the article which you can find linked above, the raid boss has been released following the "immense success of the recent Go Global Catch Challenge which regular readers of this site will recall (or can re-read) I was somewhat puzzled by the amount of Pokemon to catch being met with any form of ease, however entirely wrong Pokeémon Go players smashed past the required 3 billion catch mark and as promised new Pokemon were released worldwide (hence why I now have Farfetch in my Pokedex...)
At the time of writing the information was data mined, however this has gone live (as I have since received the message myself) and Ho-Oh can now be taken down in raids but keep in mind it is a legendary so you may want to grab a few friends rather than running and gunning solo as the legendary bird won't be an easy takedown.
On .a final note also from the Pokemon Go hub article both the regular (if such a thing exists for a legendary) and shiny versions have been found in the game so hardcore players will no doubt be looking to get their hands on the Legendary Fire Flying Pokemon .. in shiny versions
So what are your thoughts on the big bird's release? Ho-Oh is held in high regard in the Pokémon world much like it's polar opposite Lugia. What other legendaries would you like to see released next? Have you caught one? (screenshots in comments! more so if you have a shiny!)