Hi and welcome.
Many people advocate Mayday's set. My opinion is that tilesets (or character sets) suck. The support isn't there for a proper tileset yet. One thing that IS supported, however, is a graphics set.
Since it seems like what you are looking for is being able to tell the difference between creatures, a good graphic set is what you need. I currently use a Hybrid of about 6-7 different sets. It's got a pretty broad set of graphics shamelessly ripped off of other people's efforts. I suggest you take a base of what is out there right now and work from there. Most of my base doesn't even exist anymore, and I kind of cobble mine together with the old stuff I have and occasional new stuff I pick up. (Sphr's stuff features prominently in the graphic set I use)
It -Looks- like Chariot's set is a good starting point to create your own hodgepodge mess. You can find it
Here.
Alternatively, you can look
here for things you like and put together your own. You can also cobble one together out of stuff you find on the internet
Or, if you are a good sprite maker, you could make one from scratch. Any which way, Chariot's set makes a good framework for even that.
For the time being, I, like some others here, recommend staying away from tilesets. Such things replace letters in words currently. Eventually that won't be true, but until then, better to wait. If you must have one though, Mayday's combined pack is seen as the "king" around here. It is found
here. Plenty of Tilesets can be found
here. Many of them might be a perfect balance of what you want versus intolerable letter replacement.