The admin_comment_types_dropdown filter allows you to modify the options supplied to the comment types drop down in the Comments screen. The built-in values are Comments and Pings.

It is evaluated in WP_Comments_List_Table::extra_tablenav() in the wp-admin/includes/class-wp-comments-list-table.php file.

admin_comment_types_dropdown accepts a single argument:

1. array $comment_types An array of comment types to be used as options in the drop down.

Example:

There is no (useful) example for this filter yet. Have an idea for one? Submit an example.