I'm guessing that a rubber newt is a model newt (fake) made of rubber. This is the only interpretation that occurs on Google. Here are some fascinating quotes from Google:
...females were given a choice between a scented and unscented rubber newt model.
...newts will clasp rubber newt models as if they were females.
lobs slime-covered rubber newt at NewGuy
A rubber newt was used for the segment of the newt going through the air when the jar falls, when the actor supposedly throws it...
A plastic horse, a rubber newt Half an egg of Silly Putty Flash cards that I didn't study A Junior Mint I dare not eat A well-worn copy of Teen Beat...
A NEW(T) and improved site with tips, tricks and more puns than you can shake a rubber newt at...