Slack has a :seahorse: reacji, and is what I was picturing; I frequently try to use emoji that turn out to be reacji-exclusive (or reacji in the wrong workspace that I learn that way aren't Slack defaults) - I wonder if those insisting it exists are thinking of that.

Oh or Snapchat/TikTok/Instagram video/etc.? I think I've seen clips of whichever of those with overlaid stuff like seahorses.