There’s a limit of Bones for Mobile which is at 75. Instead of failing, skeletal meshes with higher number of bones just do not get rendered.
You might check logs for messages like this:
LogSkeletalMesh: Warning: Skeletal mesh SkeletalMesh /Game/Lady_01/Lady_01.Lady_01 has a LOD section with 94 bones and the maximum supported number for feature level ES3_1 is 75.
!This mesh will not be rendered on the specified platform!
At least for the Desert Dragon - this has 96 bones… not sure about Fey…
There has been some discussion on raising this limit here