"El Pato Merlín" became famous enough that someone filed a trademark application for it. The creator was not that someone.
This is the single most common way creators lose control of their own work in Mexico, and it is entirely preventable.
First to file, not first to create
Mexico grants trademark rights to whoever files first. Being the author of the character, the person who made it famous, or the one who has used it publicly for years does not by itself defeat an earlier application. The law does provide remedies: article 173 of the Federal Law for the Protection of Industrial Property blocks registrations sought in bad faith, and a registration obtained in bad faith can be cancelled. But those are contested proceedings that take time, cost money and require evidence you may not have kept.
What the creator should have done
Filed the name as a trademark the week it started to travel, in the classes where merchandise and entertainment services sit. Registered the character design with INDAUTOR to establish authorship with a date. Kept dated evidence of first use.
What you should do
If anything you have made is gaining traction, file now. The application costs a fraction of what a cancellation action does, and it is the difference between defending a right and trying to prove one.