Q.Is Yucca Plants Poisonous?
I have pet rats and one loves playing with the fallen leaf from Yucca plant, but are they poisonous?

See this article: https://www.gardeningknowhow.com/houseplants/hpgen/keeping-your-pet-safe-identify-poison-plants-in-your-house.htm
"Typically, when dogs and cats ingest yucca, it results in mild vomiting and diarrhea.", from another article.
I would suggest you ask your vet just to be certain. Some sources say yucca is not harmful to dogs and cats, other sources say it is to some degree. I was told once that the difference between a dog/cat and a mouse/rat eating mouse/rat poison was that dog/cat could vomit it up and the rodent could not.