Argyll and Bute is Scotland's most fragmented mainland region, made up of long peninsulas and a scatter of islands, so ferry crossings and long single-track roads are simply part of normal life here, not an exception. Buying or viewing a pony often means factoring in a ferry timetable or a genuinely long drive down a peninsula, and that is worth accepting from the outset rather than being surprised by it.
The reward for that remoteness is genuinely low population pressure on land and a hardy, low-key pony-keeping culture producing tough, sensible ponies used to real weather and isolation. This is not a county to search casually from a distance — commit properly to the travel involved, or focus your search on the more accessible eastern parts of the region near Helensburgh and Loch Lomond.