"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.
When two people claim the same creation
Mexico grants a trademark to whoever files first, so prior use on its own does not win the registration. What prior use can do is support a claim that the later filing was made in bad faith, which is a ground of refusal and of nullity under article 173 of the Mexican Industrial Property Law, but it has to be proved with dated evidence of the earlier relationship or knowledge.
That is the practical lesson: the dispute is decided on documents. Dated drawings, invoices, briefs, messages agreeing who would own what. Collaborations that never wrote any of it down are the ones that end here.
Copyright runs in parallel and separately. The drawings are protected from the moment they were created, whether registered or not (article 162 of the Mexican Federal Copyright Law), and where the work was commissioned the economic rights belong to whoever commissioned it unless agreed otherwise (article 83). Those two questions can have different answers, and often do.