Sorry, just now seeing this so apologies if this has been covered already or is widely known.
Here's what I heard from an Ole Miss grad a couple of years ago. I guess there was a movement on campus to move away from the "Rebels" mascot because it was not politically correct given current times and even seen as racist with it's ties to the Rebel Flag and the civil war. So the university administration put the new mascot to a vote and of course most of the college students didn't want to change mascots so in a mockery of administration and the voting process the students voted the "Land Shark" as the new mascot. It was a joke.

The administration realized it was a mockery of the process and threw away the results. The Ole Miss defense still thought it was funny and adopted the Land Shark as the unofficial mascot of the defense and started the shark fin above the head thing after big plays. That was a couple of years ago, maybe 5, who knows. Not sure if it's true, but it's what I heard.
No idea what is going on with the Ole Miss mascot today.