How can I use `-i` option from ssh-keygen?

Question

I was trying to get a public key in Openssh format from an existing .p12 certificate. I can see little to no documentation on this.

I saw this option -i option in ssh-keygen man page. That sounds similar to what I want to achieve. i.e., change one format of key to OpenSSH format.

I tried multiple examples, but I always get format not supported error. What are possible interchangeable formats?

It would be great to get an example how -i format works.

Thanks in advance.