Some people don't care for boundaries or authority, one of those people is Deadpool. The comic book character doesn't abide by any rules, he constantly breaks the fourth wall and addresses readers. In the live-action movie starring Ryan Reynolds, Deadpool continued on his fourth wall breaking trend.
Now, Deadpool is hijacking trailers for X-Men: Apocalypse and doing what he does best, making it all about himself. Ryan Reynold's made an appearance in the X-Men: Apocalypse trailer as Deadpool to give fans a rundown on what they might have missed in the X-Men trailer… Especially the release date for Deadpool in Japan.
One of the most notable lines from the trailer is "Not everyone makes it to the end of FOX movies," because apparently FOX movies aren't good or something.
Basically, FOX, the company behind both films, took the X-Men trailer to be an opportunity to get two trailers in for the price of one.