The public is being urged to be vigilant when adopting pets to stop illegal dealers cashing in during the coronavirus pandemic.
With people spending more time at home during lockdown, animal charities have seen inquiries to rehome pets increase dramatically.
As further restrictions aimed at stemming the spread of Covid-19 have been introduced in recent weeks, the demand is continuing.
Prices for dogs and cats being sold online have also soared dramatically.
Colleen Tinnelly, development manager at the USPCA, said people should ensure they can cope with the demand of pet ownership as well as avoiding illegal dealers.
She told the PA news agency they are worried about a potential influx of abandoned and vulnerable animals after the pandemic.
"Whilst we recognise that there are many kind families out there ready to give a rescue animal a loving home, there are many who do not take into full consideration the amount of time and dedication needed to look after these animals," she said.
"There is training involved and it does take up a lot of time. We really would urge the public to be very, very careful and to do their homework.
"The questions you should ask yourself is how much will it cost per week to feed and look after a dog or a cat? Have they decided what breed, what sort of house they live in, do they have a garden? Will the dog shed hair, does anyone in the family have an allergies? The dog may grow to a large size, and how much exercise will it need?
"But, most importantly, what will happen to the dog when restrictions are eased.
"So, just like Christmas, an animal is not for lockdown, it's for life. They are not an impulse buy so we urge the public to think about this quite carefully."
Ms Tinnelly said illegal breeders are trying to cash in on the demand, describing it as "incredibly worrying from an animal welfare point of view".
"The demand has created a massive hike in the prices for animals - puppies that we would have seen being sold for £300 are now being offered for £1,800," she said.
"If you are thinking about getting a dog, there are a number of checks to do - see the mother, go to the house and check they are registered breeders.
"Illegal breeders are now using Covid-19 as an excuse not to see the mum, as you can't enter other people's households. But we would still urge people to be vigilant and not to fall for these tactics.
"Do not buy your puppy from the side of the road, do not meet breeders in another town.
"If you are seriously wanting to get a pup or dog, be careful and wait. Do not go online and get the first pup that comes up, that can come with horrifying results. A lot of animals we have seen coming through have had to attend a vet and some have even died."
The USPCA works against illegal puppy farms through the UK and Ireland-wide Operation Delphin.
In August it helped rescue two adult dogs and 10 puppies at Cairnryan port.
The animals had been transported through Northern Ireland from Dublin to be sold in Great Britain.
They were in poor health and the majority had been diagnosed with parvovirus.
After intensive treatment by the USPCA veterinary team at the charity's animal hospital in Newry, the majority survived and have been returned to the Republic of Ireland under the care of the Dublin Society for the Protection and Care of Animals (DSPCA) for rehoming.
The USPCA is pushing for more regulation of online sales in a bid to curb this growing problem.
Ms Tinnelly said: "We believe that heightened regulation, or even bringing us into line with Great Britain, will help significantly."
She added: "We are very worried about the long-term impact Covid will have on animal welfare in Northern Ireland. We are bracing ourselves for an influx of abandoned and vulnerable animals if a sense of normality returns in the coming months.
"Some people may find that they now are back to work, are able to go out and socialise with their friends, can go on holiday etc, and then what? We could potentially be faced with lots of animals receiving inadequate care or worse."