Donald Alexander MacKenzie wrote a book Buddhism in Pre-Christian Britain in 1928, in which he compares Cernunnos to Shiva in his manifestation known as Virupaksha, for instance. Many authors have discussed the similarity over the years, so it isn't the case that no one noticed. You aren't alone in this. Check out Wikipedia, the images you've posted are there too, with sources.
Pashupati is another one of Shiva's titles, and means the cattle lord (hence the horns). Shiva is also known as the master of animals, and is often shown surrounded by animals, in the same way the figure from the Gundesrup Cauldron is shown surrounded by them. The pictrure of the Indus Seal from 3000bc which you posted is thought to be Prajapati, the creator god of the Rig-veda and otherwise known to shamans as Lord of the Animals. Apart from the one image of Cernunnos on the cauldron, magnificent as it is, there are only a few other mentions of the name on inscriptions, none of which offer further enlightenment. http://en.wikipedia.org/wiki/Gundestrup ... red_Figure
Cernunnos means the horned one, and may refer to horned god OR a horned headressed shaman. If you look at the stag antlered shaman/god cave paintings which pre-dated indo-european beliefs, then look at post indo-european Cernunnos from the Gundestrup Cauldron. It seems clear to me, and many others, that indo-european beliefs of the pre-celtic peoples, met european shamanic traditions, and melded, giving us the same horned shaman or god as before, but now portrayed in yogic postures and with yogic accoutrements, and is why it is so similar to the yogic postured portrayals of Shiva/Vishnu/Pashupati/Virupaksha. There are many other gods/shaman from many cultures portrayed this way, so a common source in the mix of shamanic and yogi practice seems to be the cause.
The visualilsation of humans morphed with animals is a neuralogical stage the brain goes through, which shaman under the influence of hallucinogenic drugs often experience, and which gives rise to many shamanic beliefs and practices, such as shapeshifting. The horns rising may also be representative of the energies you raise in yogic practices. Which is why this imagery appears in the art representing gods and shaman in many cultures, all around the world. The god/shaman known as Cernunnos, from the cauldron, may just be postured in the same way, going through the same practices, without being an actual representation of Shiva, or any of these other gods or goddesses.
This European stone age cave painting is usually referred to as The Sorceror:-
This is a picture from an early study on Siberian Shamanism, so the practice was still known into modern times:-
Herne and the greenman seem to have been connected with Cernunnos only in recent times, perhaps only due to a similarity in the names noticed by 20th century pagans, who connected the two when Cernunnos was shortened to Cern. There's no older connections between Herne and Cernunnos. Herne is connected to vegetation, though, and Cernunnos to animals, so the connection was tenuous, at best.