The list of horn makers spans all time, and not all still exist.
Andreas Jungwirth Atkinson Brass and Company Briz Horn Company Buescher Band Instrument Company C.G. Conn Christopher Cornford Daniel Rauch Dieter Otto Ed. Kruspe Engelbert Schmid F. E. Olds Finke Gebr. Alexander Hans Hoyer Herbert Fritz Knopf Holton Kalison KHS Musical Instruments Klaus Fehr Lewis & Duerk Lukas Horns Jacob Medlin, Apex, North Carolina Otto Patterson Hornworks Paxman Musical Instruments Ricco Kühn Stomvi Thein Woodhead Horns Ltd, UK Yamaha Corporation
References
sv:Lista över tillverkare av bleckblåsinstrument