I de-nicotine things by soaking them in a bucket of Simple Green cleaner, changing it out every few months until the item washes clean again. Yes I did say MONTHS.
Nicotine staining is probably one of the most disgusting things ever, but I say that because I had to renovate my house after it had been lived in by smokers for 30 years. We had to use razor blades to peel the film off the windows!!! EW.
Ponies are very absorbent as they are vinyl...