So , what did it grow into a Chayote? That's what it looks like

It does look like a chayote, definitely not a mango.

I don't mean to insult you, but have you eaten mangos before? If not, was this "mango" fruit green and starchy or yellow-orange and fibrous?

Any chance the seeds or pots got switched?

I'm surprised this thread is still alive. :-)
Unfortunately the seedling is not alive. The heat got to it and it died.

I am however forced to agree that this was not a mango. I love mangoes and eat them all the time. I will never know how I mistook this for a mango.

As it grew a little it really did look like the leaves of a chayote .

Place embarrassed face here ===>