You can, but when you use a slash in the url rewrite you may have to add a <base> tag to the html headers on the pages to identify the correct directory for the page so the css and images display properly if you use page relative urls.
The update to the htaccess would be:
RewriteRule ^timeline/[a-zA-Z0-9_.-]+\/([0-9]+)$ horse-profile-timeline.php?horseid=$1 [NC,L]