I have no idea who made that horn. It's a nice one, though. I don't think I've ever seen scrimshaw lines that deep and broad before, so I would say it is unusual in that respect. I have some visual issues and couldn't make out the letters clearly. I thought it said "Maquakota River Valley," and I searched that term, but Google found "Maquoketa River" instead. This is evidently a tributary of the Mississippi, in northeastern Iowa. The name is an English corruption of a Meskwaki term, "Maquaw-Autaw," meaning "Bear River.
I thought I saw the word "Butterfield" on there, too. I found a Butterfield Creek in Illinois, but I don't think it connects with the Maquoketa.
Map horns are fascinating. I think they may be most meaningful to people who have some personal, family, or historical connection to the area depicted, though, and this may affect the value. So, maybe this horn was made by or for someone who lives near the river depicted on the map.
There is a state-level blackpowder shooting association in Iowa. I don't specifically remember their name, but they do have a website. There's a nice article and photos of an original Hawken rifle owned by one of the members on the website. You might look them up, and submit your question and photos to them.
Good luck, and thanks for showing that interesting horn!
Notchy Bob