Skeletons cannot control the skeleton horses. They are on the horse just unable to move.
MCPE-17521
Skeleton traps- Skeletons cannot ride or move on the horses
Resolved
Fixed
2
(Unassigned)
36239
Confirmed
Phone - Android - Samsung Other (Specify in description)
5.1
skeleton, skeleton-horse-trap, skeleton_horse
0.15.90.8, 0.16.0, 0.16.1, 0.17.0.2, 0.16.2, 1.0.2, 1.0.4.11
1.0.6.0
Retrieved
As of 1.0.6.0 you can ride & tame skeleton horses from /summon & spawn eggs. I have been unable to find a naturally-spawned skeleton horse from a skeleton trap, however, so I'll hold off resolving this ticket just yet.