The Nymphs presided over various natural phenomena--from springs to clouds trees caverns meadows and beaches. Worshiped by the ancient Greeks and said to come from the earth Nymphs were seen to be care givers of the land and life in general. Although not immortal they lived extremely long lives.
Nymphs are sometimes compared to fairies - they were unpredictable a little scary and often showed up in folktales. However they are also entirely different. Nymphs are all women and this is definitely significant in the "scary" aspect.
One of the things to notice is the common theme of women's sexuality being translated as scary while women's chastity is seen as good.
As they are most generally pictured as young vital beautiful young women embodying the age of fertility they were strongly attractive to males.
With this in mind it should not shock you that these somewhat scary spirits are at their scariest to mortal men when sex enters the picture
Due to their sexual freedom women who had "clinically concerning" sexual appetites were considered nymphomaniacs. Nowadays doctors prefer the term "hyper sexual" which can refer to both men and women.
The male counterparts of the nymphs were the Satyrs Panes Potamoi and Tritons.