I'm using K2 for my website to show horses and i would like to make a link in an item to another horse.
Herfore i would like to have the possibility to choose the other horse by an extra field dropdown.

Would anyone know how this is possible?

I don't think is possible with extrafields (without having to c/p the link).

There is a K2 extension for the JCE editor which allows you to directly link K2 items by generating a list of them.

Thanks for your reply. I know it's possible by creating a link in the item article, but i would love it if this was possible through a extra field dropdown choice. The website must be user friendly possible as this must be maintaned by multiple people. Maybe this is a good feature request :-)

Thank you for your feedback. We will consider it

There are however several extensions which offer related content functionality, you can always try one. Try searching at the JED or at the extend K2 section of this site for K2 plugins that offer this sort of functionality.