When you lend your male pig to another owner, it's called "outside rental"
You can choose the "outside rental" by selecting "pig plaza" after putting your grown pig onto the truck.
If you want to change the setting after sending your male pig to the plaza,
click "male pigs" shown on the lower right side of the screen in the plaza.
If you lend your pig to another owner, you can receive rental fees.
Rental fees vary on what type of pig you lent.
After a certain time, you'll be able to ship the pig you were lending.