As a high school French horn player I am wondering whether there is a brass instrument with a range similar to the alto saxophone. Although horn and alto are usually given the same parts the horn has a more tenor range (from about D2 to Eb5) and anything above Bb4 is a real struggle for me, whereas alto players can easily go up to F5 or higher with little apparent effort. Is there an brass instrument with a range similar to the alto saxophone?
There are a few, but as you might have guessed, they are not terribly common.
The alto trombone is pitched a perfect fourth higher than a tenor trombone. The range is usually considered to go from A2 to B♭5. Its primary use, as far as I'm aware, is in classical music from the late Classical & early Romantic era.
The tenor horn (so named in British English, but also called the alto horn in American English) is halfway between flugelhorn (slightly more common) and a baritone horn (rather more common.) It is somewhat more common in the UK than in the USA, due to its use in brass band music. Its gamut is approximately A2 to E♭5.
The mellophone has a range from approximately A2 to E♭5 as well (slightly higher if it's a mellophone in F rather than a mellophone in E♭.) It is often used as a substitute for French horn in marching bands.
The closest I could find is the alto trombone. http://www.orchestralibrary.com/reftables/rang.html
The tenor trombone also goes up to F5 if you just want to get up to there.
The way I understand it, French horn is itself an alto instrument, or at least one that more often than not plays within the alto range. As evidence of this, consider the fact that, when composers write chords for the brass section to play, the horns are usually given notes between those played by the trumpets (soprano) and the trombones (tenor). Furthermore, just as some trumpet players can play nearly an octave higher than the instrument will ever be called to play in orchestral settings, giving its range an upper bound more comparable to that of other soprano instruments, some horn players can play up to a high E on piano, which is just as high what other alto instruments (like viola) can play.