Capcom just announced they'll be revealing a brand new beast for Monster Hunter World: Iceborne later this week. This monster is slated to be a "fan-favorite," but there's little else to go on right now.
As per the tweet from the official Monster Hunter: World Twitter account just below, "Developer Diary 5" will be airing later this week on March 21. These developer diaries usually involve three members of the development team outlining new content, including a look at brand new monsters arriving in the game very shortly.
We know two of these monsters will be Furious Rajang and Raging Brachydios. These two ferocious alternate forms of the regular beasts will be arriving for all Iceborne players just a few days after Developer Diary 5 airs, on March 23. We should be getting a look at both beasts in action, and probably a sneak peek of both their respective armor sets.
As for the "fan-favorite" monster that'll be revealed during the developer diary, it's anyone's guess. The announcement in the tweet above does state that the monster will be arriving in Iceborne in May however, so we've got well over a month to get repeatedly murdered by Raging Brachydios and Furious Rajang before this new monster arrives.