I mean, its not like emojis were always standardized. It is completely possible that there was a "emoji" or "emoticon" of a seahorse in a messaging application. I wouldn't be so quick to accept that your memory is incorrect.
Oh or Snapchat/TikTok/Instagram video/etc.? I think I've seen clips of whichever of those with overlaid stuff like seahorses.
(You're absolutely right!)
E.g. no one seems to be misremembering a sea cucumber emoji or anglerfish emoji - but there are other alleged emojis such as swordfish or bandit/bank robber, where people have the same reaction:
https://www.reddit.com/r/MandelaEffect/comments/haxgql/can_w...
It would be interesting to see if LLM behavior is also similar. E.g. if you asked about an anglerfish emoji, would they straight-up tell you it doesn't exist, but for swordfish would start to spiral?