My friend, you're in for a potential world of pain...

The square-taper crank setup was the new kid on the block in the 70s; rendering cottered cranks obsolete it was AKA 'cotterless'... it certainly had no shortage of its own headaches, but it was light-years ahead of cottered cranks.

I turn up my nose at em both; I'll only ever deal with that stuff on other people's bikes...

Bearing life might only be 25% of what you can get with square-taper, but everything else is so much more improved, especially the wrenching aspect.

With the older 3-piece setups you could sometimes hit a snag that'd bail you up for an hour or more, but with the modern 2-piece crank / external BB deals, they just reliably come apart with an allen key. Sweet.

Anyway, getting back to damn cotter pins, you might be lucky... try this:

Unscrew the nut so it's sitting just proud of the top of the cotter pin, and give it a sharp whack with a hammer, as straight-on as you can. Don't hit it too hard (sometimes those pins are made of a steel resembling plasticine, and you're trying to move the pin by its threads); just try to hit it square so as not to bend it. (You leave the nut on and hit that instead, because otherwise you'll damage the thread.)

If it doesn't give in the first few tries, don't persist with the same methods - they can be properly stuck - you're much better off bringing the proper tools to bear before any damage has ensued.