How many Rabbi's are there that are named "Rashash"? I can't seem to find any sources that speak of another rashash, besides the ones mentioned below. I was told that there was one that lived a thousand years ago. Who is that person?
I know of the Rashash of Shalom Sharabi and Shtrason.
I was told that there was on who lived before the Arizal the one who invented the Kavanot that the Arizal uses in his book.
It is possible that the person who told you this misunderstood, Rabbi Shalom Sharabi (the Rashash) commentated on the works of the Ari, zl, and wrote a siddur (Siddur Ha-Kavanot) known for its Kavanot, that is used by many kabbalistic jews; the Ari never published a siddur.
As far as I know the main ones are the two mentioned in your original post. Rabbi Samuel Strashun is best known as a commentator on the Talmud. Rabbi Shalom Sharabi was an 18th Century post-Lurianic Kabbalist. Depending on whether you are in a Talmudic context or a Kabbalistic one usually one of those is usually called "the Rashash."
In some circles, Rashash can also refer to the eighteenth century Yemenite poet Rabbi Shalom Shabazi. Finally, I have on occasion seen Rabbi Shimon Shkop (19th-20th century talmudist) abbreviated in writing as Rashash, though I have never heard him called that in conversation.
As for "I was told that there was one that lived a thousand years ago," it would not shock me if there was some Rishon abbreviated Rashash (e.g., Rabbi Shimon of [some place starting with sin or shin]), but I am not aware of one and couldn't find one one Google.
As for someone who invented kavanot used by the Arizal, it ssound like a confused description of Rav Sharabi, as BID points out in their answer. Rashash Sharabi was post-Arizal and built on his kabbalistic system. It's also possible the person you were speaking to mixed up the Rashash and Rashbi. Rashbi refers to Rabbi Shimon bar Yochai, a rabbi in the Talmud later treated as a mystical figure and author of the Zohar. The Ari believed the Zohar was written by Rashbi and refers to him often.
