According to Famitsu Magazine (make sure you have google translate handy) the official release date for the first Monster Hunter: World DLC has been announced. On March 22, 2018 players will finally be able to take on this new content. This first major DLC for Monster Hunter World and will be the introduction of the new monster, Deviljho.
The Deviljho is described as (according to Monster Hunter Wiki): "Deviljho is a very large, bipedal Brute Wyvern characterized by its uniform forest green coloration and muscular upper body". It is also know to eat it's own tail because it is so hungry for food.
Along with the new monster the DLC will bring with it a lot of new weapon balances and tweaks. You can see the full list here, The update will also add the the ability to adjust the look of your hunter. It talks about this being a free service at first but will turn into a paid service in the summer.
Another big change which should have been in the game at release, is the ability to now choose where the player wants to return after a hunt. So now a player can choose to return to the gathering hall after each hunt instead of having to load into the main town and then reload back into the hall. Right now the gathering hall was one of the most under utilized areas in the game, when it should be the place to be when in a multiplayer game.
The official announcement is still coming from Capcom on March 15th, and I will cover it here.