Why can't I use my choco mount yet.... and for that matter where did my horn go?

[Weks]

It seems I don't have my horn for my goob mount nor can I even use any mount is this a bug or do I have to wait till start day ?

If you can't find an item, then there's a few places it may be...
1) Inventory (double check)
2) Armory Chest
3) Calamity Salvager npc
4) Retainer(s)
5) Storage chest in Inn room.

If you can't find a mount, then check to see it you used the summoning item already. It will be listed under Mounts in the menu. You will then have to add it to Crossbar.