Developer Niantic has recently updated their Raid Bosses for the current season, and legendary Pokemon Ho-Oh is no longer available in Pokemon Go as a result.
The company seems to be favoring the first and third generations of Pokemon, as both Ho-Oh and Lugia have been removed, while Kyogre and Mewtwo remain in place. It wouldn’t be too surprising for Rayquaza to make a come back here soon.
With this change, as well as others, the full roster of Raid battle bosses is as follows.
Five Star Bosses
Four Star Bosses
Three Star Bosses
Two Star Bosses
One Star Bosses
You may want to strive for Mewtwo while you still can. There is no certainty as to how much longer they’ll be available.