Hi,
Ridable Horses can't pass through TerrainTriggers (https://wiki.facepunch.com/rust/Volume_Prefabs). Modular cars can, workcarts can (with this plugin: lone.design/product/work-cart-spawner/), but Horses can't. I have looked arround for any collision trigger in RidableHorses but I can't seem to find one that works. The trigger used in the workcart plugin seem to be workcart specific, so no luck there. Any help is appreciated!