I saw the word (tyger) instead of (tiger) in a poem. But I couldn't find that word in any dictionary, and I guessed that it may be another form for (tiger). Are these two vocabs synonyms? If so, why did they write this vocab in that way???
They are synonymous. "tiger" is the modern spelling, "tyger" an the Old English spelling.
Read more here about why William Blake, the poet, used "tyger": http://www.lcc.gatech.edu/~broglio/eromantic/tyger.html