The second paragraph below should answer your question, Izzy. It comes from Wikipedia:
"The Tyger" is a poem by the English poet William Blake. It was published as part of his collection Songs of Experience in 1794. It is one of Blake's best known and most analyzed poems. The Cambridge Companion to William Blake (2003) calls it "the most anthologized poem in English."
Most modern anthologies maintain Blake's choice of the archaic spelling "tyger". It was already "slightly archaic" when he wrote the poem, he spelled it as "tiger" elsewhere, and many of his poetic effects "depended on subtle differences of punctuation and of spelling." Thus, his choice of "tyger" has usually been interpreted as being for effect, perhaps to render an "exotic or alien quality of the beast", or because it's not really about a "tiger" at all, but a metaphor.