The 2014 Anniversary event Polytrinic Acid Horta combat pet no longer shows on any of my characters promotion tabs. This was an unlimited unlocked that i have had and used on my other characters for years. I did have the exact same issue with one of the Klingon T6 Battlecruiser Promotions that also vanished for a long while but i was never bothered because i never had a Klingon character however that promotion did come back on its own after a long while. Hopefully the Acid Horta promoton will do the same.
I contacted support and it is thought to be a bug and asked me to write a bug report here.
"We've conducted further investigation and tested possible workarounds for this issue but it looks like this is a result of a possible bug. While we don't have a fix for you, we'll gather the details that you've given to us and pass them on for further investigation. Still, we strongly recommend submitting a report in the official bug reports forum."