This might be a bit confusing, since my memory is pretty vague and TV channels in my country sometimes reused or moved shows between networks.
I remember a series that aired on Jetix or Disney XD (though I could be mixing that up). It was about a boy who encounters a red monster that came into the human world because of some kind of organization. The boy ends up befriending this monster.
The monster had the ability to disguise itself as a human and blend in with people. Meanwhile, that same organization kept bringing other monsters into the human world, trying to find or capture the red one. From what I remember, it followed a “monster of the week” format.
I don’t remember much else, but one detail that stands out is how I associate the red monster’s design with something like a chubby, horned character—kind of similar (in my memory) to Ralph from Wreck-It Ralph. I might be mixing things up with other shows from that time, though.
I’m pretty sure the series was 3D animated, with that early 2000s style. I likely watched it sometime between 2008 and 2012, but I’m not completely certain. I do remember that it didn’t air for very long in Mexico, it just kind of disappeared from TV at some point.