I have read differing opinions about poets using a thesaurus or not in writing their poems. Some think that it is unauthentic to use anything but your own language. For those, that is authentic for them.
I use Roget’s Thesaurus. It is more of an idea generator for me. But the word that is picked must serve the poem, be welded in, so as not to stand out by itself and ruin the flow. Unless of course that is the objective.
I also use Merriam’s Dictionary in concert with the thesaurus, to make sure of the exact meaning I wish to convey. I was working on a poem today and thought the word adjure would be good until I looked it up and it did not convey the message I was trying to get across.
The poet's eye in a fine frenzy rolling, Doth glance from heaven to earth, from earth to heaven; And as imagination bodies forth The forms of things unknown, the poet's pen Turns them to shapes, and gives to airy nothing A local habitation and a name. -William Shakespeare's A Midnight Summer's Dream