It looks like Latin endings on familiar nonLatin names ... could be plural but if its in a genealogy it's more likely to be a genitive ending. Which just indicates possession. For instance, imagine three cousins all called Louise. In the family it'd be convenient to say: I saw Elizabeth's Louise in town, and she says Mary's Louise is learning Latin! In which case, stick on some Latin endings and you get Elizabethae Louisa, Mariae Louisa and so on. Same with Frederick. Frederick's Mark and Robert's Mark would be Frederici Marcus and Roberti Marcus.
Try that, and see if it fits your data. It doesn't of course mean that you had ancestors who actually answered to the names of Elizabetha and Fredericus - but the person who wrote them down was either being very precise and accurate, or a bit of a show-off.