3 Letters:
* pop
* lap
* pal
* oar
* pro
* rap
* lop
4 Letters:
* polo
* role
* rope
* poor
* pair
* pray
* peal
* pull
* loop
* opal
5 Letters:
* parole
* purple
* poulp (rare word for octopus)
* loopy
* lupus
6 Letters:
* poultry
* popular (the original word itself)
7 Letters:
* lopulate (rare word meaning to grow densely)
This is not an exhaustive list, and there may be other valid words depending on your dictionary or word game rules.