If you are looking to volunteer in Guatemala, I recommend ARCAS.  ARCAS was originally created as a rescue center to care for and rehabilitate wild animals that were being confiscated on the black market by the Guatemalan government.  Since its establishment, the ARCAS Rescue Center has grown into one of the largest and most complex rescue centers in the world, receiving between 300 and 600 animals of more than 40 species per year.  

ARCAS offers three plans for volunteers:

1) Plan Mochilero is for experienced travelers who speak some Spanish and prefer to arrive at the project site on their own with no reservation.
2) Plan Reserva offers a reservation, and for the Peten project airport pick-up.
3) Plan Completo is for less experienced travelers who don´t speak Spanish. For an additional deposit, volunteers are guaranteed a reservation, picked up at the airport and offered additional services depending on the project they are going to.
When you arrive at the project site, you will be asked to register and sign a waiver form. Please bring with you a copy of the picture page of your passport.
Other than the Plan Completo deposit, you can make your volunteer payment in cash in quetzals or US dollars at the project site. (Plan Reserva and Plan completo need to be made beforehand) 
You can contact ARCAS at