Fox's mutants will appear in a teaser at the end of Sony's spider-sequel.
In a surprise move, a short scene from the upcoming blockbuster feature X-Men: Days of Future Past will play during the credits of The Amazing Spider-Man 2. The scene will reportedly be part of an action sequence featuring X-Men characters Mystique, Havok, Toad and William Stryker; and is regarded as strictly an advertisement for X-Men, not as the beginning of a "crossover" between the two series.
What makes the arrangement unusual is that movie studios seldom collaborate on promoting eachother's films, particularly when they are expected to compete with one another at the box office (X-Men: DOFP will release only one month after ASM2.) The move is likely to be seen as rival studios working together toward the common goal of undercutting Disney's Marvel Studios; which has used shared-universe continuity between it's superhero films for an unprecedented string of box-office hits. Spider-Man and The X-Men are both Marvel properties, but currently reside with other studios because of rights deals made before the veteran comics publisher established its own studio.
It appears unlikely that this marketing collaboration would lead to an actual "team-up" between the two blockbuster series; but it's an open secret that every studio in Hollywood is looking for ways to steal (or muffle) Marvel's thunder: The shared-universe conceit allows the studio to brand even risky ventures like August's Guardians of The Galaxy as psuedo-sequels to one of the biggest action franchises on the planet, which threatens the box office prospects of many other studios and would only increase in dominance were they to reacquire major players like Spider-Man or Wolverine. So it's not too hard to imagine Sony and Fox (which has a famously toxic relationship with Marvel leadership) calling truce to hold-fast against the bigger "enemy."