Given that 20th Century Fox's upcoming X-Men: Days of Future Past merges the casts of the original X-Men trilogy and the 2011 prequel X-Men: First Class, fans will get the best of both worlds as past and present versions of these mutants are featured under one cinematic roof. Today, we have word that a secret cameo has been revealed, so if you don't want to be spoiled by this new information, stop reading now.

SPOILERS AHEAD!

Digital Spy is reporting that X-Men: The Last Stand star Kelsey Grammer will have a cameo appearance as Hank McCoy, a.k.a. Beast in X-Men: Days of Future Past.

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Here's what an unidentified insider had to say about the cameo, which will occur towards the end of the film as the actor playing an "alternate adult Beast."

"Producers had the brilliant idea to bring back Kelsey for Days of Future Past. It wasn't hard to get him to sign on for a guest appearance. He jumped at the opportunity the second it was brought to his attention."

It isn't known how this "alternate adult Beast" will differ from the character he portrayed in X-Men: The Last Stand, but it seems that the future timeline was altered somehow in the story. The cameo appears to have taken shape late in the game, with the appearance shot on the 20th Century Fox lot during post-production, under the guise of a separate project.

"Kelsey's special cameo was secretly shot during post-production on the Fox lot under the impression that Kelsey would be on set to film a pilot for a different project."

Nicholas Hoult plays the younger version of Hank McCoy, revealing in an interview last month that Hank now has the ability to transform in and out of his Beast mode, which, in essence, makes him a blue version of Marvel's Hulk. Here's what Nicholas Hoult had to say during that interview.

"What's happened up to this point is between the time of the last movie and this movie my character has created a serum which basically controls his mutation so his appearance is normal as long as he doesn't get worked up. Any animal instinct or urges, that kind of brings him out. So yeah, he changes into Beast a few times throughout the story and they've done some great action sequences with him this time, particularly in the mansion flying around on these chandeliers and stuff."

It's possible that, since the younger McCoy developed this serum in the past, it has altered McCoy's future, leading to this alternate version of Beast played by Kelsey Grammer.

Of course, this cameo has not been confirmed by 20th Century Fox yet, so stay tuned for more developments.