Check here for the basics:
Trumpet Peculiar Frequency spectrum
In the above answer, it’s stated that the trumpet’s “natural” overtone series of only odd harmonics is altered by the flare of the bell and the shape of the mouthpiece so that instead of being only the odd harmonics, it has essentially a normal harmonic series, but the fundamental is missing.
A pedal tone is a way to play the “missing” fundamental. Any resonant system has a lowest resonant frequency and for bell and mouthpiece brass it’s there, it’s just not as easy to sound as the first harmonic and much of the series above it.
It’s also a different embouchure from the rest so pedal tones aren’t as frequently taught because many believe learning to play then might create bad embouchure habits in the student.
One thing about the above linked answer I’d be careful of is the reference to the clarinet, which does only have its odd overtones but is also a conical bore instrument so I think there are some subtleties beyond the half closed cylinder model at work in the clarinet.
There’s a Wikipedia article on the topic also.
Everything I’ve found suggests the concept of pedal tones applies to all members of the brass family, namely cylindrical bore metal instruments with a flaring bell and hemispherical mouthpiece that are primarily played by sounding an overtone series along with valves that change the sounding length of the primary cylinder. The fact that the horn uses a different mouthpiece shape doesn’t exclude it from the category.