BatMUD reduces the experience value of monsters that are constantly being killed. If you personally kill a monster, it will be worth moderately less experience for you for a while. This is called your personal tune on that monster. It will also be worth a little bit less experience for every other player until some time passes. This is generally referred to as the global tune. You can see the total of these two variables by using the consider skill at a monster.
Try to avoid killing any monster more than once per day if all you’re after is experience. If it’s a cash or equipment monster, then of course you may want to kill it whenever it’s available to slay, but you’ll note that you get very little experience compared to its difficulty.
Where this gets complicated is in what counts as the ‘same’ monster. Some monsters are ‘clones’ of each other, and killing one reduces the value of them all. Common signs of this are the monsters having the same or very similar names and being in the same area. Some wizards create a unique individual monster for a group of monsters with the same name though, so the only way to know for sure is to pay close attention and keep track of things with the consider skill.
The reason for this mechanic is to keep players from camping the same areas over and over again. Before this existed, it wasn’t rare for high end players to for example stomp all the Duzes in duzelton, all the guards in Shadowdale, and all the animals in Eowyn, and never leave those three areas.