Mounting is considered casting a spell, so it will reveal you like any other casting action. You can however do clever stuff like run on an ethereal. Cast invis. Target yourself. Dismount and then stealth away quickly.
As designed by OSI back in the day they created the mounting action as a spellcast. Code: public override void OnDoubleClick( Mobile from ) { if (Validate(from)) { new EtherealSpell(this, from).Cast(); } } Custom servers over the years have removed this to suit their own tastes, but as Renaissance is a classic server based on the mechanics from around the time of Publish one we stick to the old rules. It provides good balance with other mechanics for it to work this way. Might not be the preferred method for everyone but at least you can earn your ethereal here. As I understand it on most other servers you have to either pay $$ for one, or give gold to someone who did. So they are more likely to make them more . . over powered so that they increase their cash sales.