Δ or δ are the true greek delta sign. (As far as I know) The "delta" on Mega Rayquaza's head looks kinda like \'/.
Look at his head in the picture:
You see the \'/, right?
Why is it like this? After all, it is the delta episode where you catch Rayquaza, you would expect to see a delta on his head instead of a \'/. (Whatever it is)
>As Mega Rayquaza, its body becomes longer and gains more features. Its head becomes longer with the Δ symbol appearing on its forehead.
Look at it upside down. The circle and the line through the triangle slightly looks like δ, and the triangle is Δ. Rayquaza always has its head bent down so it appears to be upside down. Or, it isnt meant to be a Δ and Game Freak just made a random design. Who knows.