Let me just give some quick clarification. The Alatreon encounter will not be available in Monster Hunter World: Iceborne until May. However, by the time you read this today, a title update should be downloading. This free update today adds the Furious Rajang and Raging Brachydios encounters. Furious Rajang is tailless and has a new pin attack. Raging Brachydios is much larger than the standard version and exploding slime is everywhere. Rounding off the update are both free and paid content updates. For full details, watch the below developer diary.
Monster Hunter World: Iceborne is available now for PC, PS4 and Xbox One.