Miscellaneous Objects are scenery objects that do not fit into any of the existing secery themes. In the RCT2 custom design scenerio editor, you can make a Miscellaneous Object category if you select the "Advanced Scenery" button on the upper right hand corner of the ride/attraction/scenery selection box. Then you can select any specific scenery as you wish (max 152).
The Misc, Object Scenery box usually should be found as the last box on the scenery theme tab as a red question mark tab.