Is there a legal presumption about which neighbour owns a fence built exactly on the property line?

Not really. Ontario doesn't have a clean, bright-line statutory presumption stating that a fence built exactly on the boundary automatically belongs equally to both neighbours, or to one over the other. Ownership and responsibility for a boundary fence tend to follow more practical factors instead: who actually built it, who paid for it, and whether there's any agreement, formal or informal, between the neighbours about it.

Where the history is unclear or contested, nobody remembers who built it, or the properties have changed hands since, there isn't a fallback statutory ownership rule that simply settles the question on its own. What Ontario does provide is a process for the practical question that actually tends to matter more: the Line Fences Act's fence-viewer mechanism, which resolves disputes about building, repairing, and paying for a shared boundary fence going forward, regardless of exactly who's considered to "own" it historically.

If you're in a dispute over a fence built on the property line, focus less on trying to establish abstract ownership and more on using the Line Fences Act process to resolve the practical questions of maintenance and cost-sharing, since that's the tool Ontario law actually provides.