My fanon: When Angel Bunny died at the ripe old age of fourteen, one of his descendants presented herself as a replacement for her ancestor’s position of assistance. Fluttershy accepted the doe, and Fluttershy’s friends were glad that Succubus* was as gentle as Angel had been bratty.
*Named for the Night Mare’s, Luna’s dream-commandoes, whose wartime duties are to deny the enemy restful sleep, as well as promises of good treatment for any wise enough to surrender. In later centuries (but still before her fall) the term started to acquire a sexual connotation for what those mares got up to when granted a visit to the dreams of their husbands, who were often guards on Equestria’s borders.
Edited because: Footnote from my pre-existing fanon.