Since Beta4, Horde3D is licensed under the Eclipse Public License v1.0 (EPL).
The EPL is a quite liberal license and has less restrictions than the popular LGPL. Basically it allows you to use Horde3D in free and commercial projects as long as you contribute improvements like bug fixes, optimizations and code refactorings back to the community. The EPL allows static linking and is not viral, hence it does not affect any other modules of your application.
Please read the exact license agreement before downloading any code.