Well, did Led Zep rip off a rather dull number by an obscure band to produce Stairway to Heaven? If so, I’m quite glad they did, since the original is quite dull and soporific whereas Stairway to Heaven is an enduring work of genius. But plagiarism is fraught with complications. Creative people often have similar ideas at similar moments; there is a sort of cross-pollination in the air which also affects other things such as scientific discoveries. To be sure, the chord-sequences running through both songs are eerily similar (and in the same key) but does that prove plagiarism? You’d have to prove first I guess that Led Zep heard Spirit’s song, which they say they ‘don’t remember’ doing. It could be true, then again perhaps it’s a convenient way of covering up the truth.
But here’s the thing: we all take from each other. Every poet I’ve ever read or heard has given me something – how could it be otherwise? Yet why would I bother plagiarising any of them? No disrespect, but I’m quite capable of writing my own original work, thank you – just as Led Zep were quite capable of writing original songs.
But there’s nothing new under the sun. That chord sequence is probably in a hundred pieces by Bach: it’s what you do with it that counts. Led Zep took it and made it genius. So if they did plagiarise the original song I’m quite glad – because the original was utterly boring.
These cases are ultimately about money. And in the end the only winners are the lawyers. Anyway, listen and compare:
Kirk out
Sarada Gray 
Spirit were not an ‘obscure’ band: anyone with a more than superficial knowledge of late sixties/early seventies rock has heard of them. They were Zep’s support act on a UK (and, I think, a US) tour.
I frist heard Taurus in 1987 and didn’t ‘hear’ the similarity to STH – that had to be pointed out to me, but it’s – at most – a very slight resemblance and, as you infer, uses a chord sequence which is several hundred years’ old.
It is atypical of the band’s sound and I’d strongly recommend exploring their oeuvre in more depth. They made four great albums between 1967 and 1970.
Zeppelin – more particularly Jimmy Page – do have form in this area, though. Google ‘Jimmy Page, Jake Holmes’ for a real tale of grand larceny!