These are quite common in Yucatan, Mexico.

The leatherleaf slugs belong to the family Veronicellidae. This particular one could be Sarasinula plebeia, but it is not so easy to definitively ID these.

I originally identified this species a few years ago from the description on this website, but since then they have added an update stating that my original source is also unsure on this one.

UPDATE: It seems that IDing certain slugs by pictures isn’t a good idea. In 2024 when pictures on this page were uploaded to iNaturalist, another user suggested a different species in the genus Leidyula, and then user “deneb16,” a mollusk specialist at UNAM, Mexico’s main university, added the comment that all Mexican species of the family this slug belongs to can’t be identified without dissecting their sexual organs. The family, she agrees, is the Leatherleaf Slug Family, the Veroncellidae.

So, I am not 100% of the species, but it is a leatherleaf slug.