Mechanically speaking, a leather cannon is most like a cannon. It’s anti-infantry instead of anti-heavy infantry and light cavalry. It’s also countered by culvs and other anti artillery units. As a cannon, it does kinda replace the Skirmisher for Sweden, but that doesn’t make it a Skirmisher.
The bonuses and range aren’t really important to what the unit is. It’s a light, maneuverable artillery unit which only kinda replaces a Skirmisher. They don’t counter light cavalry, and are countered by things which counter artillery. They get upgraded by artillery upgrades in the Arsenal as well.
Edit: btw, a lot of different cannons have different bonuses.
I think this was already pointed out in this forum. Napoleon gun is a 12-pounder canon-obusier.
Age 4 Field gun is also a 12-pounder gun (maybe lower).
Heavy canon is a siege gun (32- or 24-pounder), whereas horse artillery is probably a 6-pounder gun.