The lizard Holcosus undulatus is a common, widely distributed, species in Central America.
This species was recently (2017) moved into the genus Holcosus; it was formerly known as Ameiva undulatus.
Twan Leenders, an accomplished herpetologist and author of the Guide to the Amphibians and Reptiles of Costa Rica (and some other books on reptiles and amphibians in Costa Rica), helped me identifying this species.
I met Twan during my visit to the Biological Station Rara Avis (now an Eco Lodge) in the Braulio Carrillo National Park in Costa Rica in 2003 and ever since I sometimes ask his assistance in identifying reptiles and amphibians that I have seen but not been able to 100% identify.
Los Tarrales Natural Reserve, Patulul, Suchitepequez, Guatemala, 18 January 2020