What is "Holech Al Gachon" referring to?
Rashi: It refers to a snake, which crawls on its belly. 1
"Gachon" means 'bent over and low' (Rashi)
Which four species is the Pasuk referring to when it writes (a) "Kol Holech", (b) "Holech al Arba", (c) "Kol", (d) Marbeh Raglayim"?
Rashi: The Pasuk is referring to (a) worms (and similar species), (b) scorpions, (c) beetles (and similar species) and (d) centipedes, which have legs from head to tail.
Why is the Vov in "Gachon" big?
Moshav Zekenim #1: It is the middle letter of the Torah (Kidushin 30a). 1 It hints to the Torah, which was given on the sixth of Sivan. [Torah] is called "Aruchah me'Eretz Midah". Therefore Vov is in the middle, for the Torah was proper to be given at the end of 2000 years from creation, had Avraham not said "ba'Meh Eda." 2 It should have been in the middle of the  years of world, excluding the [2000 years of the] days of Mashi'ach. 3
Moshav Zekenim #2: It alludes to six orders of the Mishnah, six directions (including up and down), six days of Creation, 6000 years of the world.,. It is all in the merit of the Vovim at the top of [almost all of] the Amudim (columns of text in the Torah).
Moshav Zekenim #3: It corresponds to six occurrences of Hashem's name in Tehilim 19 - "Toras Hashem, Pekudei Hashem, Edus Hashem, Mitzvas Hashem, Yir'as Hashem, Mishpetei Hashem", and the six words in "Shma Yisrael Hashem Elokeinu Hashem Echad", and in Baruch Shem Kevod Malchuso l'Olam va'Ed.
Moshav Zekenim #4: Holech Al Gachon alludes to the snake, which was cursed "Al Gechoncha Selech" (Bereishis 3:14). Through it fell six good gifts that Hashem gave to Adam ha'Rishon (Bereishis Rabah 12), therefore it was decreed that it be in the form of a long Vov (without arms or legs), and later Hashem will return [the gifts].
Daf Al ha'Daf (Kidushin 30a, citing R. Y. Alter): It is about 5000 letters after the middle letter! Rather, it is the middle of the 17 special letters (they are big, small or not read). Machzor Vitri (522 b'Sof, citing R. Yosi Tov Elem) lists about twice as many.
This was at Bris Bein ha'Besarim. If the Torah is in order, this was after "Lech Lecha" (when Avraham was 75 years old), in the year 2023. Those who disagree say that it was only five years earlier, so Yetzi'as Mitzrayim was 430 years later. (refer to Shemos 12:40:3:1)! Did Hashem delay Matan Torah because He knew that Avraham will say so later?! (PF)
Moshav Zekenim citing Avodah Zarah 9a: The world is 6000 years - 2000 of Tohu (void), 2000 of Torah, and 2000 of the days of Mashi'ach. (Presumably, the days of Mashi'ach are excluded from the calculation, for Mashi'ach was proper to come when they began, just sins have delayed him. (PF)