When merging mplayer with the xanim USE flag -- which pulls-in media-video/xanim as a dependency -- it still can't play videos that use xanim-based video codecs (such as Indeo or Cinepak). The reason seems to be that when you try to play a video that uses an xanim-based video codec, mplayer tries to load a codec from /usr/lib/xanim/mods/*, but errors-out because it can't find it. bug #181959 describes this issue. I'd suggest removing or masking the xanim USE flag because it doesn't do anything useful, at least amd64.
*** This bug has been marked as a duplicate of bug 181959 ***