It seems an obvious point to make, but the major recurring motifs in horror posters are unnatural creatures and murderous people. In older film posters in particular these are made very obvious and fit well with in your face text and exclamatory taglines, whereas in more recent posters they are suggested- hidden in the darkness, only partially present, or not depicted at all as the poster shows just what they are capable of, or a more abstract idea of them.
In terms of colour the horror genre tends to feature a large amount of red and black. These colours are appropriate to the genre with the connotations of death, fear, and blood. Often the posters feature some shade of green, this in itself can be seen as a spooky rather than murderous colour and hence approiate to the more supernatural films of the genre.
In older posters the typography is very over the top. Some kind of bold, rash type face is used to slam the title to your face, this is often highlighted in some bright green, yellow, or red, and more often than not features a exclamation. The genre has certainly developed to become more subtle andsuggestive with type now merging into darkness, or being very faintly playing on the fear of the unkown.