To some extent, pine may have been used simply because it was available. It was used in shipbuilding, but also other construction--and to some extent as firewood, or gathered to distill out pine tar, or just burned to get rid of it. I doubt it was used in the belief that the inherent resin would seal the wood better. Folks then were probably aware that oak (white oak) was used for barrels that held liquids, so the resin wasn't necessary. Pine (white pine, not long-leaf pine) is easy to carve with a knife; poplar, cottonwood, willow, and basswood are all more-easily carved with the same tool.
Pine pitch or pine tar was used, in at least some instances, for sealing horns in the west, and to seal maritime horns such as those used on-board ships or by seal hunters. I don't know if it was commonly used to seal horns made commercially, or by private citizens, etc but it certainly could have been as it was widely available.
At a minimum, to do the horn work you need a knife and an awl (to drill holes). (Now that I think about it, I think I see a way to drill the spout with a bow drill. I may have to try that.) Additional tools make the job go faster, or the outcome likely to be better, but they aren't absolutely necessary.
The spout end can be removed just like cutting a sapling with the same knife--make a cut around the spout, remove shavings down to the bottom of the cut, then repeat til you are through. The large end of the horn can be cut in a similar manner. The spout can be drilled with an awl, or heated wire. I've seen native-made horns done in just this fashion, as well as some I'm sure were made the same way by farm kids.
Scale removal can be done quickly with a knife by splitting off the scaled layers (works better if you boil the horn first, and work while it is hot). When thinking about shaping the horn, remember that scraping is a finish operation: carve the horn in order to shape it, then scrape.