Summary:
Entities have restrictions on riding non-mountable entities when using the /ride command. For example, you cannot make a passenger on an ender dragon. Only certain mobs can become passengers or mounts.
This doesn't match Java where you can now ride any entity using the /ride command.

Steps to reproduce:

1. Try to ride with the /ride command on an entity that cannot be ride without commands.

   /ride @s summon_ride cow

Observed results:
You get an error when trying to ride a non-mountable mob.

Expected results:
You start riding on the entity specified in the command.