Let's take the demonstrative pronoun "eso". Use it as a direct object (i.e., put it in the accusative / after a transitive verb), and it stays the same, right?

No creo eso.

Now, take the demonstrative pronoun "ellos". Turns out, we have to change it to 'los' like so:

¿dónde los viste?

So, both "eso" and "ellos" are demonstrative pronouns, but the latter does inflect and the former does not. Is this directly comparable to how the English demonstrative pronoun "it" doesn't change in the accusative, but "they" does? Or is there something else at play?