"It is a puppy."

Even though "puppy" is masculine (and "dog"), if the puppy/dog in question is a female dog, can the word be changed to feminine? In Spanish, it's taught that "dog", "puppy", "cat", etc. are masculine nouns, but then in real life, if you have a female animal, you can change the word to feminine.

Why is "Este o cățel" wrong? I thought I wrote it down from another sentence I'd seen before plus "o" was one of the options for translating "a" from English. Thanks!