Before the invention of plastics, horn was used for many of the same things. And it could be used as a "water tight" container. The classic use is as a cup - either with the full point on it (you just can't set it down until it's empty), or with that point cut off and a wood plug glued/tacked in one end. Whittle a removable wood plug for the end, and you have a container for tinder, your fire kit, salt/spices, a sewing or fishing kit, personal items, etc. They also would cut it into sheets, press them flat, and use them in place of sheets of glass in candle lanterns - hence that old word Lathorn. And then there was buttons, toggles, spoons, shoehorns, "butter" knives, women's stays, and so many other little items that are now often made from plastic.
Horn is pretty easy to work with.
First, a caution. Try not to breathe the dust from sanding/grinding horn. It is organic matter, and could lead to a nasty sinus infection if you breathe too much of it - just like breathing in most any large amounts of dust.
The outside and inside of your horn can be scraped with most any knife or chisel. And it can be cut with a regular hand saw, but a hacksaw gives you a little better control. A regular file will smooth up and shape horn. And it will go slow enough to control. Different grades of sandpaper work for the final smoothing up. A strip of sandpaper held at each end and then pulled back/forth over the horn works real well to smooth it up. Often the hardest part of working a horn is just hanging onto it.
With some heat, you can bend/shape horn. Hot to boiling water will soften it, and then it can be molded to shape. A strip of softened horn pressed between two spoons and left to cool, will give you a table spoon - after you trim the outside edges and sand it smooth.
With that flat piece on the bottom, it looks like it probably was hung on the wall and used
to hold flowers (also called a Wall Pocket). They can also make a nice small bird house. But attached in pairs onto a hat/helmut would lead the neighbors to wonder if you were ... going a Viking!
The possibilities are only limited to your imagination. But the simple beginner projects to consider would be making a cup or a storage container.
Have fun with your ... project.
Mikey - yee ol' grumpy blacksmith out in the Hinterlands