/ride allows a mob to mount anything it can normally mount. But if you try making a skeleton mount a spider through /ride it said it can't mount the target. But skeletons can mount spiders.

Steps to reproduce:

1. spawn a skeleton with a spawn egg

2. do /ride @e[type=skeleton,c=1] summon_ride spider

Expected result:

The skeleton should get a spider mount.

Actual result:

It says the the ride was summoned but couldn't be added to skeleton.