Board Thread:General Discussion/@comment-27215513-20151114155308/@comment-25692211-20151114170053

Well, the MCU rights belong to Marvel and the X-Men rights belong to Fox. A loophole in the contract means both studios own Quicksilver. However, Fox doesn't own Scarlet Witch and Marvel doesn't own Magneto. Therefore, Fox used their Quicksilver to have an important role as Magneto's son, while Marvel used their Quicksilver to serve as a sidekick of sorts to Scarlet Witch.