The Zohar literature, including the Zohar, Zohar Hadhash, and the Tiqquney HaZohar – along with their respective books and sub-divisions – was published over the course of almost 300 years (approx. 1300-1587 CE) and straddles the periods of the late rishoniym and early aharoniym; with the era of the former generally held to have been during the 11th to 15th centuries, and the era of the later generally held to have been from the 16th century until the present time.
Although there was much written on the subject of the Zohar and the authenticity of its content, only a minority of what is extant was authored in the narrow window between the publication of the Zohar literature and the end of the period of the rishoniym. Much of what exists in this genre was written in the period of the early aharoniym and remains very valuable. The main reason for the lack of earlier literature is that the Zohar, even after its initial publication, was not a very widespread or well-known book.
What is available exists in two types:
 independent works authored specifically on the subject of the authenticity of the Zohar literature, and  quotes from Hazal, Geonim, and Rishonim (e.g. Rasag, Rashi, Tosafoth, Rambam, et al) whose explicit statements in times prior to the Zohar are directly contradicted by, and in many cases preclude, explicit statements made later by the Zohar and its commentaries.
Works in the period of the [later] Rishonim written disputing the Zohar and its authenticity:
- Sefer Behiynath HaDath – Rav Eliyahu Del Medigo (15th Century CE)
- Sefer HaYuhasiyn, account of Rabbi Yishaq de-min Akko – Rav Avraham Zakuto (15th Century CE)
Works in the period of the Aharonim written disputing the Zohar literature and its authenticity:
- Sefer Ariy Nohem – Rav Yehudah Aryeh DeModena (17th Century CE)
- Mitpahath Sefariym – Rav Ya`aqov Emden (18th Century CE)
- Shu”T Hatham Sofer (6:59), referring to the work of Rav Emden – Rav Mosheh Sofer (18th Century CE)
- Teshuvah Me-Ahavah (1:14) – Rav Eli`ezer Fleckeles (19th Century CE)
- Milhamoth HaShem – Rav Yihya Shelomo Al-Qafih (19th Century CE)
These are by no means exhaustive lists, but they do comprise the majority of what is available.
The following are examples of literature prior to the publication of the Zohar which discuss similar topics:
- HaNivhar Emunoth wa-De`oth – Rav Sa`adyah Gaon (10th Century CE), This work is a comprehensive compendium of explanations not only of the position of Torah Judaism on hashqafic topics, but also includes the arguments of detractors and the basis for their being rejected. The interesting thing about this work is that it deals with almost every major theme of the later “Kabbalah” which became embodied in the Zohar literature and rejects them as not being authentically Jewish. These topics include the idea of multiplicity or aspects as relates to the One Transcendent God, Reincarnation, and Emanation (assilyuth אצילות) among others.
- Moreh HaNavokhiym – Rav Mosheh ben Maimon (Rambam – 11th Century CE), a work also detailing the necessity of intellectual and rational approaches to the Torah and the Prophets, as well as explaining the meaning of many of the misswoth and the various reasons behind them. It also deals with concepts later embodied in the “Kabbalah,” such as “secret” mystical names of God and amulets, which are roundly rejected.
- Ma’amar Tehiyath HaMethiym – Rav Mosheh ben Maimon (Rambam – 11th Century CE), In the first section, the Rambam accounts for the misunderstanding of his own teachings regarding the resurrection from the dead by bringing an example of a gross misunderstanding of God’s own words in the Shema` (Devariym 6:4). He refers to the “belief of the dualists” who believe that the three mentions of the Divine Name in the Shema` (i.e. HaShem, Eloheynu, HaShem) are three separate forces/entities/modes of the Divine (halilah wa-has) which supposedly comprise a composite unity. The Rambam of course rejects this reading of the Shema` in his statements there. However, the Zohar (2:53b) espouses just such a heretical interpretation. This passage was ironically used by later Christian Hebraists and the Catholic church in justifying the supposed validity of their belief in a “Trinity” from “Jewish” teachings.
- Rashi and Tosafoth on b.Meghiyllah 9a (11th, 12th-13th Centuries CE), In an interesting passage about the request of King Ptolemy (Talmay HaMelekh) that the hakhmey HaSanhedriyn write for him a copy of the Torah in Greek, the Gemara explains that several deliberate changes to the text were made by them while translating in order to avoid certain polytheistic errors. Two of the notable changes were made to Bereshiyth 1:1 and 1:26; the former change being that instead of the text reading “Bereshiyth bara Elohiym” they wrote “Elohiym bara bereshiyth,” and the later being that in place of “Na`aseh adham” they wrote “E`aseh adham.” In the first instance – since syntax in the Greek language which often puts the most important noun in the sentence first and sorts out the meaning and parts of speech via case endings – the hakhamiym did not want the Greeks to think that “Bereshiyth” was the name of one deity which created a second deity named “Elohiym” (halilah) and that there are thus multiple powers in Heaven (halilah wa-has), so says Rashi. The Tosafoth add to this by saying that “Bereshiyth eyno shem kelal ela ba-tehiylah” meaning that the term “bereshiyth” is not a name at all, but is rather just the Torah’s way of saying “In the beginning.” The second change was made due to the presence of the plural form (i.e. “Let us make man”), lest again the Greeks think that the Torah promotes polytheism and that multiple gods created mankind (halilah – see Rashi there). However, the Zohar – in commenting on these very passages – adopts the mistaken and erroneous views which these changes were specifically intended to negate. On Bereshiyth 1:1 the Zohar says that “Reshiyth” is the name of one of the partzufiym/sefiyroth and it creates another/emanates another partzuf/sefiyrah named “Elohiym” which it can then inhabit. On Bereshiyth 1:26, the Zohar depicts two of the partzufiym (faces/personalities which supposedly make up the Divine), “Abba” and “Imma,” arguing whether or not they should make man – “Abba” is con while “Imma” is pro – and in the end “Imma” says that although mankind will sin against us “Let us make man” anyhow. The implications of these interpretations in light of the Gemara and its commentators are wide-reaching.
There are many more things which could be listed here, but much of it is already written in the works mentioned above.