To cite zoo in publications use:

  Zeileis A, Grothendieck G (2005). “zoo: S3 Infrastructure for Regular
  and Irregular Time Series.” _Journal of Statistical Software_,
  *14*(6), 1-27. doi:10.18637/jss.v014.i06
  <https://doi.org/10.18637/jss.v014.i06>.

A BibTeX entry for LaTeX users is

  @Article{,
    title = {zoo: S3 Infrastructure for Regular and Irregular Time Series},
    author = {Achim Zeileis and Gabor Grothendieck},
    journal = {Journal of Statistical Software},
    year = {2005},
    volume = {14},
    number = {6},
    pages = {1--27},
    doi = {10.18637/jss.v014.i06},
  }