Horoscope sites all around the web going back to the early 2000s have been spreading the idea of the connection between the mythological labors of Herakles and the zodiac. The idea had some popularity in the 70s as well, but I've traced this back to "The Key of Destiny" published in 1919 by so called Christian mystics, who had spawned these ideas from their readings of Porphyry and Hesiod (which I've been unable to track down thus far). In actuality, we find the surviving origins in Orpheus.
"Now as Herakles is celebrated in this hymn1 as the Sun, and the Sun is the same with Hyperion, the reason is obvious why Herakles is called Earth's best blossom. Herakles in this hymn is said to shine with primogenial fires; since he is no other than Protogonus2 in the intelligible and intellectual worlds, and the Sun in the sensible world. Or in conformity to Orphic theory, it may be said that he is celebrated with solar epithets, as being one of the satellites of the Sun." Thomas Taylor, in his translation of the Orphic Hymns.
1. 12th Orphic Hymn to Herakles
2. Another name for Phanes, the ancient Greek primeval deity.
I harbor my own disdain for the Christians who propagated this into their occultism, which contained passages such as "The nine heads [of the Hydra] represent the cycle through which the monster of perverted sex force crawls and pollutes mankind. The more we turn our thoughts toward sex, even for the purpose of conquering it and cutting off its heads, the more it grows..." Slaying the Hydra is Scorpio's labor, and Scorpio is inherently connected with sex, which is evil... you get the idea. Nonetheless, the actual ancient concept of Herakles being connected to the sun remains of interest.
Herakles struggles with the Lernaia Hydra, killing Cancer sent by Hera in the process; the labor of Scorpio. For every one head cut off, two more grew back. Socrates says, as written by Plato: for every argument refuted by Euthydemus and his brother, they grew two more. A constant cycle, worthy of comparison to Scorpio's transformative and regenerative aspect as a Water sign. Another point: the constellation Ophiuchus is immediately nearby to Scorpio in the sky. A traditional attitude explained that Ophiuchus wrestled with the Scorpion, even stepping on it.
If it were any other beast, Herakles could kill it. But he must capture without harm the Ceryneian red doe of moon goddess Artemis; hold it close to his heart, and bear it to the altar. This is the labor of Cancer: the crab which was slain.
Swarms of iron razor in mire must be flushed, birds of metal sacred to Artemis or Ares; killing and trapping do not work against their feathers and poison. Crashing cymbals at twilight— clashing bronze krotala forged by Hephaestus— does. Three deadly birds remain which Sagittarius must eliminate with its own bow and arrows. Cruel speech, self referential speech, and casting pearls before swine.
Like the Stymphalian birds of Sagittarius, the labor of Gemini was partially brought down by arrows poisoned during Scorpio's labor. The Hesperian Dragon, never-sleeping serpent which tears entrails, was slain by Herakles in order to reach the golden apples.
POSTSCRIPT
Through this exercise, we realize that there is no discernable pattern with the signs as there exists in astrology; adding question to the veracity of this imagined symbolism of Herakles representing the sun as it moves through the signs in the sky. There is no pattern which Herakles follows other than the orders he receives from King Eurystheus. However, several inferences may be made at these instances of "success feeding into success." These are my own original conclusions.
The labors of Leo and Taurus are central pieces to the order; both being fixed signs, and the first and seventh labors respectively. The second labor was that of Scorpio, forming a square aspect; the king refused to acknowledge it. After the labors of Cancer and Libra, the labor of Aquarius was similarly refused by the king, with Aquarius and Scorpio forming a square, and Aquarius being the opposite of Leo.
There is significance to be found in Capricorn's status as the final labor, additionally symbolic in its constitution of the capture of Cerberos. Throughout antiquity, Capricorn was considered a gate of the gods; a spiritual gateway. Here, Herakles approaches the gate of Hades himself.