I hate cats, I love displacer beasts. They have above animal level intelligence but aren't overly smart, are the size of a lion/tiger, get missed 50% of the time, have long reach and good hit points. They come in numbers from solitary to pride (10 or so), which gives you all kinds of opportunities to throw at the party.
I like to foreshadow the presence of a displacer beast with a dead or gravely injured blink dog. Most of the time, however, the players never catch on to what is lurking in the shadows until they come face to face with the beasts. So when they aren't paying attention, usually while in camp or in a fight with something else, I will have them catch a glimpse of something out of the corner of their eye. Then when the opportunity presents itself, the ambush happens.
If you missed the previous link to how lion prides stalk prey, I highly recommend it for learning basic squad-level ambush techniques. For some reason, I always seem to have displacer beast encounters at night, maybe due to their color, maybe due to wanting heavy armor on the ground instead of on the PC, and maybe I read something in an old MM - I don't know. But on a moonless night in the forest, with little to no breeze, you can expect a displacer beast attack at some point during the watch of a human or other low-light vision lacking party member.
In the newer editions (I'm not sure about the older editions), displacer beasts also get a bonus against ranged attacks. So keep them at range if possible, using their tentacles to trip the melee types. Use their speed (and leaping ability which I think they should have, given 6 legs and all) to get around defensive positions and characters. If you want to use hit-and-run, then let them get a halfling/gnome/familiar/animal companion and get out. This encourages the players to immediately start the tracking, don't forget night modifiers and sleep penalties for spell casters.
The other way I like to use displacer beasts is as pets for various humanoid baddies. Their size makes them acceptable mounts for the smaller goblinoids (technically all the goblinoids, but that just doesn't seem right to me). And I have been known to put them in place of dire wolves/boars when working in conjunction with ogres/hill giants/trolls.