A horse is a vehicle and Stargate should attempt to handle it. However, I believe the workaround for getting vehicle teleports to work between worlds is to remove the entity and create a new one. This should work fine with minecarts, since they're all the same. (he even includes special handling for storage minecarts) Horses are not the same, so there'd need to be special handling to basically recreate the same horse at the destination. I recommend turning off handleVehicles until either horses are filtered out or specially handled in Stargate.
BTW, post the whole error next time.
