I'm working on a novel which is based on a true story. I first thought of writing from the heroine's POV, but later changed my mind and introduced a pet in the plot as the narrator of the story. Which means the story and the whole incident is told from the pet's POV (it being one of the eye witnesses). But while writing I'm often times overwhelmed with keeping up with the pet's antics alive in the story and concentrating on the plot. In a few scenes, I end up giving more importance to the pet's narration instead of the story line. For me both seem important, but I'm unable to keep up with both at the same time. Where am I going wrong?

I would suggest the main points to bear in mind are the mindset of the species as oppposed to our own. Of course, being human one cannot know the thoughts of animals.

Dogs are more of a we species than we are. If the pet is a dog, the humans would be either pack members and leaders or members of other packs and potentially threats. Territory is another thing that is important to dogs and they will protect as much as they can lay claim to regardless of the actual size of the territory they are supposed to defend.

Dogs rely on smell and hearing much more than sight. Humans will be identified by scent from a distance.

There was an experiment in the UK where a dog owner was sent to town to spend an undetermined amount of time there before returning and an observer watched her dog. The dog’s behaviour was noted and recorded. The owner returned and was asked when they had decided to return. The dog, as soon as the owner chose to go home, became excited.

Cats are more difficult. A cat might choose to help a human and can be fiercely loyal, but they are more of a me or us species. Cats will bring their kill to us - why? Is it to prove they are good hunters? Is it to share the bounty since we, clawless and helpless, cannot hunt?

I might add a tinge of pity to the cat’s outlook on humans. Poor furless things too helpless to catch a mouse and too dull witted to accept a gift when given.

As usual, I have an example from Worm! In the Interlude for Arc 4 (main chapters are from the heroine's 1st-person POV, interludes each have a different character in tight-third), Rachel's dog Brutus tells the tale: https://parahumans.wordpress.com/category/stories-arcs-1-10/arc-4-shell/4-x-interlude/

The actual story is that Rachel goes to break up a dog-fighting ring and frees all of the dogs, without telling any of her teammates. This comes into play later, as basically the Gang With Dogfighting then perceives this as an Attack From Rachel's Gang.

But Brutus? He doesn't quite get all of that.

Master and man talk for a while, and Brutus waits because Brutus is good boy. Not paying attention to what they’re saying because of smells. Bad smells. Sounds of dogs yelping and barking from inside the door. Then Master says “Stay” and man starts touching Brutus. Touches like vet touches, not like Master scratching.

Brutus doesn't use our grammar, but his own is consistent. He knows certain things, he tries to guess other things based on the bits he knows (Master/Rachel is angry? He growls to emphasize her anger - hopes it's ok.)

Master is kneeling beside cages and dogs inside smell like blood and poo. But dogs aren’t angry, aren’t moving. Brutus nuzzles master with nose and lies down beside Master and Master wraps her arms around Brutus’ neck. Master hugs Brutus tight for very long time.

Cars that make howling sounds start to come from far away and Brutus makes little barks like Master taught him. Master gets up and takes Brutus into the car and gets in other door and the car starts moving.

There is a focus on specifics - "cars that make howling sounds" not "police sirens." Things are focused on scent first. "Master" always has his attention, no matter what else is going on. (Oh, this chapter makes a little more sense if you realize this is a super-powers universe, and Rachel's power is that she can make dogs grow REALLY big temporarily.)

I had to read it more than once to get that it was more than Brutus Goes Walkies, that there was plot-focused stuff there, but if your entire book is from this POV, or this POV is frequently used, then the reader should fall into it. (Interludes in Worm are always a little challenging, since we don't know the PO usually.)

I definitely recommend reading other pet-narrated works, to note their strategies. Bunnicula comes to mind.

No need for antics? Other ways to illustrate the nature of the narrator.

Do the pets antics need to play a role in the story? Having the pet as the narrator wasn't your first thought so I'm assuming the antics are not super important. Don't get sidetracked from your plot unless you now do want antics for some reason (i.e. don't have antics for the sake of antics, or to make the animal narrator more realistic - there are other ways to illustrate "catness" or "dogness").

What if your cat/dog is super old/lazy, not a lot of antics going on but a lot of observation. You could also just have them as a narrator without explaining the antics - I can narrate from the point of view of a cat without having to interupt myself all the time to talk about watching birds etc. If you want to remind the reader of the nature of the narrator just say something like

"Sarah left the room to answer the knock at the door. I hopped down from the window sill and followed, but at a distance. I was never one to like visitors..."

Without me telling you you know this is a cat speaking - sitting on the window still, and with the general reservedness common to most cats.