was the plain negative of aru once or in a certain dialect aranai?

Question

the plain negative of aru is irregular:

NOT: aru --> ara (mizenkei) --> ara+nai BUT: aru --> nai

Answer 1

Yes, there was ara-nai. Quote from 『おあむ物語』 (c. 1665):

くびにおはぐろを付て。おじやる。それはなぜなりや。むかしは。おはぐろ首は。よき人とて。賞翫した。それ故。しら歯の首は。おはぐろ付て給はれと。たのまれて。おじやつたが。くびもこはいものでは。あらない。その首どもの血くさき中に。寝たことでおじやつた。

From 『雑兵物語 おあむ物語』, page 134, 岩波書店 ISBN4-00-302451-6.

While quite short, for those interested in language history, I'd like to point out that this text is particularly rich in early 17th century colloquial speech. Other than the above negative ara-nai, there are early examples illustrating the change from bigrade to monograde verbs and merging of /di, zi/, among others.