Aphrodite is a unique monster added in SLASH'EM and UnNetHack. She is effectively a tougher to kill nymph, with the same attacks but much lower AC and more hit dice. She is also slightly faster than an unhasted player.
Aphrodite will only ever appear on the nymph level, which is generated 45% of the time, on level 23 or 24. It will have a number of random n, including potentially brownies, pixies and quicklings. Be careful if you lack see invisible that you don't run into a pixie: use telepathy to identify where they are beforehand.
In UnNetHack, her level appears significantly earlier: anywhere from level 5 to 14. However, as UnNetHack doesn't have SLASH'EM's new high-level nymphs, this isn't much of a concern.
There are always at least four rings on this level, which is quite often useful if you are missing a particular ring for your ascension kit.
Aphrodite is not actually that difficult to defeat. She can do no more harm than a regular nymph—she is simply more likely to hit you and can take a few more hits herself. By the time you meet her, though, you should have an attack wand or two. Now is a good time to use them. Even if she does steal an item, like all nymphs she will flee afterward. Use telepathy to find her and kill her quickly before she regains her courage.
I will sing of stately Aphrodite, gold-crowned and beautiful,
whose dominion is the walled cities of all sea-set Cyprus.
There the moist breath of the western wind wafted her over
the waves of the loud moaning sea in soft foam, and there
the gold-filleted Hours welcomed her joyously. They clothed
her with heavenly garments: on her head they put a fine,
well-wrought crown of gold, and in her pierced ears they
hung ornaments of orichalc and precious gold, and adorned
her with golden necklaces over her soft neck and snow-white
breasts, jewels which the gold-filleted Hours wear themselves
whenever they go to their father's house to join the lovely
dances of the gods. And when they had fully decked her, they
brought her to the gods, who welcomed her when they saw her,
giving her their hands. Each one of them prayed that he might
lead her home to be his wedded wife, so greatly were they
amazed at the beauty of violet-crowned Cytherea.
translated by Hugh G. Evelyn-White ]