Maintenance for the week of May 13:
• [COMPLETE] ESO Store and Account System for maintenance – May 14, 9:00AM EDT (13:00 UTC) - 12:00PM EDT (16:00 UTC)
Well, since the Skyfire Guar mount is available Nov.30-Dec.6 I wouldn't expect to see the December Showcase until the 6th of December. Pretty sure (if I recall correctly) that they always post it on the last day for whatever is the last item for that month after 10am EST.
I could be wrong and hope I am but I'm not going to count on seeing it before next Thursday while a November Showcase item is still available.