Otro is a simple word in Spanish that looks and sounds like its English equivalent, "other" or "another." But with this ease of recognition and use, many non-native speakers misuse otro by adding an article where it doesn't belong.
Here's a trick question. How do you say "another" in Spanish -as in "I'll have another (beer)."?
Answer: "Me tomaría otra (cerveza)."
Note that it's NOT: una otra or un otro. That's wrong. It would be like saying "an another" in English.
In the sixth installment of the short documentary Con Ánimo de Lucro, we encounter a short clip from Nicaraguan TV:
...la policía capturó a dos sujetos...
uno porque supuestamente se acababa a robar una moto...
y otro porque se metió a una casa...
"...the police captured two individuals...
one because he had allegedly just stolen a motorcycle...
and the other because he broke into a house... "
[Captions 13-15, Con Ánimo de Lucro > cortometraje > 6]
Note that once again, otro in Spanish doesn't require the pronoun it does in English.
The time to use a definite pronoun before otro is to distinguish between "another" and "the other"-- if the distinction needs to be made. For example:
Otro día = "Another day"
But: El otro día = "The other day"
So, if you add a pronoun before otro(a), make sure it's a definite prounoun (i.e., el or la) and not an indefinite one (i.e., un or una).