The Otherworld has, historically, been associated with fairies and monsters. In fact, the other reality has often been referred to as Fairyland and Elfland in folkloric traditions. It is also the Annwyn of Welsh mythology and the lands associated with the fallen gods of Ireland (see: Celtic Interpretations). However, there is no one species in the other reality that is directly analogous to our own or to a species we would recognise as Fair Folk.
During the struggle to make the study of the Otherworld respectable in the seventeenth and eighteenth century, the newly organised unnatural science academics ignored the folklore of the Blessed Isles in favour of study and the connection with the Fair Folk of various Blessed Isles traditions were put aside. However, nineteenth century academics started to put together the Second Sighted observations of spirit forms, some of which have been documented in prized Mortimer letters and diaries since the late medieval era, with the more nebulous traditions of the Fair Folk in their own lands. At the time, the main question this raised was estimating the number of people without known royal or noble lineages who had the second sight in order for these folk traditions to have been so accurate.
Given the link between Fair Folk and spirit forms, it is likely that at least some of the traditions of fairies in this reality are due to plant spirits roaming from their Otherworld physical forms. The best example of this is probably the traditions of yew trees being planted in churchyards at night by fairy women, which suggests that yews exist in both realities due to the parent trees ensuring that their offspring have their roots in both.
There is some debate about whether animals who cross from the Otherworld must choose to use only one of their animal or spirit form once they are in this reality, or if they can use both. For example, Sir Theodore Mortimer-Warren suggested in his contentious The Founding Of A Nation that the unicorn that followed Edmund I became the woman who married his brother, Roger, and thus the source of the Dukes of March’s Second Sight.