Naming Issues
There has never been an issue in the history of music that served to confuse as many people as did the naming issues of the Tenor Horn, Baritone Horn and the Euphonium.
When this instrument was first used in the UK, it was called a Tenor Horn instead of an alto as advertised in Sax's catalogue, because the cornet was concidered an alto instrument in the UK when it was called a soprano in the US. In the US, the terms 'baritone' and 'tenor horn' were used similarly to the way Adolphe Sax called them.
However in the UK, because of the different names for the same voice range, Adolphe's 'Eb Alto Horn' was called a 'Tenor Horn', his small-bore 'Bb Tenor Horn' was called a 'Baritone Horn' and the original 'EEb Baritone Horn' became obsolete. At the same time in the US, there was already a large-bore Bb instrument called a 'Baritone Horn'.
Worse still, people began to use the term 'Baritone horn' on the Euphonium when it was a completely different instrument with a very large-bore related to the Tuba. The appearance of the UK Tenor Horn looked just the same as the small-bore UK Baritone Horn or US Tenor Horn, and their names began to muddle up even further.
And yet, in other countries, there are yet more names for the Teno(Alto) Horn; for example, 'Althorn' in Germany and even 'Eb Horn'. But just remember that the Tenor(Alto) is pitched in Eb, unlike the Baritone(Tenor) Horn, which is pitched in Bb. Also, of the three Bb instruments mentioned above, the UK Baritone Horn has the smallest bore, followed by the US Baritone horn, and the Euphonium has the largest Tuba-like bore of the three. (from 'Al's Tenor Horn Page')