If there is already a support for this, pls explain properties

While it is documented here, this feature is being reworked in beta 4.

The current behavior is:

container=horse
variants=_skeleton_horse
texture=<replacement.png>

The rework will be on the dev-properties-rework branch, which will be merged into dev. This rework will be a breaking change.

The rework will still support everything (most things) OptiFine supports, but will add a new OptiGUI syntax for more advanced scenarios.

The new skeleton horse syntax is planned to be:

[skeleton_horse]
texture=<replacement.png>

Thanks for the quick reply!

You can track the progress of the rework in #32, and when beta 4 gets released, don't forget to check the documentation to learn how it works.