There is this reference in the item.php file
/*
Note regarding 'Related Items'!
If you add:
- the CSS rule 'overflow-x:scroll;' in the element div.itemRelated {…} in the k2.css
- the class 'k2Scroller' to the ul element below
- the classes 'k2ScrollerElement' and 'k2EqualHeights' to the li element inside the foreach loop below
- the style attribute 'style="width:<?php echo $item->imageWidth; ?>px;"' to the li element inside the foreach loop below
...then your Related Items will be transformed into a vertical-scrolling block, inside which, all items have the same height (equal column heights). This can be very useful if you want to show your related articles or products with title/author/category/image etc., which would take a significant amount of space in the classic list-style display.
*/
First of all you need to create K2 template overrides in order to modify this file. Read here how to setup your overrides
getk2.org/documentation/tutorials/174-templating-with-k2-and-the-concepts-of-sub-templates
Then just add the changes mentioned to this reference and you will have the related items block horizontally.
