I've made such one but it's complex because it monitors several logfiles (auth apache/error apache/access syslog, and fail2ban) to have a snapshot of the actually banned IP's.
It makes a linecount with a grep for the interesting things at a specified rate and displays the lines in the difference count.
The problem is the logrotate because for apache/error you have also apache/error.1 and so on.
Tell us what files you want to monitor, the final usage of the script and what you've already tried.
Basics :

Code:

#!/bin/bash
OLDCOUNT=0
while :
do
    NEWS=$(grep -Fw 'denied' logfile)
    COUNT=$(echo "$NEWS" | wc -l)
    DIFF=$((COUNT-OLDCOUNT))
    if [ $DIFF -gt 0 ]
    then
        echo "$NEWS" | tail -n $DIFF
    fi
    OLDCOUNT=$COUNT
    sleep 15
done < logfile

That's the simple way (one file, no logrotate, no colour, no config file)