Judging by other responses, you are not the only one who thought that.
it is a comet falling, tail blazing behind it :)
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I actually was always baffled by Summoner's icon as I always thought it a stylized lyre and mixed it up with Bard a lot... I always thought scholar was a book... And dragoon a Lance but at least I never got confused which job they were.
I always figured the SMN icon was a simplified version of the arcanima symbols in their books, since it...still doesn't look much like a horn to me.
I guess I never had issues with SMN because I love FFT. >_>
Yeah, anyone with even a tiny knowledge of past FF games should realize that the SMN Job icon was clearly a 'summoner's horn' which is traditionally part of the Job's costume in past FF games (or, in at least FF title, an actual racial physical attribute of SMNs (as in FFIX). The SCH icon was a little more obtuse though, but at least SE have finally revealed all. :)
In keeping with SMN and SCH's Job icons - one thing that still needs answering though, is exactly what ACN's class icon is supposed to be. To me, it looks like an 'armillary sphere', a mechanical device like a celestial globe that is used to chart various astronomical objects and their movements in relation to the earth (such as the movement of the sun and moon, the lunar phases, and the movement of the stars across the night sky).
How this is related to the ACN class however is anyone's guess - although I suspect the class's original form that had been planned for version 1.0 but never implemented and was dummied out (where they apparently were meant to be equipped with odd-shaped staves and laid mechanical traps in battle) might have been intended to have some lore-based connection with celestial movements. But only SE knows for sure.
In any event, I'm still very keen to see this little mystery resolved, especially as all the other class icons are fairly obvious as to what they are (and the lore that surrounds them).