It is not a mysterious bus. 7 days a week it passes through Sosua about 7 am.
If you want to go direct to Las Terenas from Sosua, the only way to do it is by taxi or rental car.
You can take that bus to Sanchez and get transportation from there to Las Terrenas. Or you can take a series of publicos or quaguas to Nagua and a Caribe Tours bus from their to Sanchez.
To return, you have the same options. The bus that passes through Sosua about 7 am leaves Samana at 2 pm. If you are waiting along the highway at the intersection with the highway to Las Terrenas about 2:15, you will be able to flag down that bus on its way back to Sosua and Puerto Plata.