WordPress custom fields check box

I'm trying to make it so the last item doesn't show a comma. When I use 'rtrim' it puts the check boxes first and the element second. Here is what I have but it shows up before element on next line up.

<?php echo '<p>' . rtrim('Good with: ',',' . the_field('good_with')) . '</p>'; ?>

result:

Dogs, Cats Good with: