One of the year's hottest retail markets is animal onesies, a fashion trend that began as kigurumi – the animal-character garb-of-choice for Japanese performers – before its adoption as street wear/sleepwear by young hipsters, celebrities and musicians.

The word kigurumi translates as "wear a soft toy" and today the androgynous fleecy all-in-ones boast myriad species and character designs including bears, tigers, lions, frogs, sharks, lemurs and, yes, chickens.

Thousands of the gargantuan garments hang in Aussie wardrobes and most traders insist there is still no sign onesie wonderment is ebbing – for now.

“We sold 6000 of these suits including six different animals in six hours about 12 weeks back,” says Gabby Leibovich, director of online shopping site CatchOfTheDay.

“Why did we sell that many? Because onesies are very hot right now. We noticed the trend, bought two more containers and they all sold out too.”

Local suppliers agree the once-obscure urban ensembles are now firmly mainstream; appealing to comfort-seeking grandparents and toddlers in equal measure.

One man recently ordered 100 as invitations to his 60th birthday party, says Daniel Labib, co-founder of Melbourne onesie outfit Kigu.me.

Labib says his company recorded gross turnover of $1.5 million in 2012-13 on the strength of about 30,000 onesie sales and these figures are projected to double this financial year.

“I could say we'll turn over $5 million this year but my business partner [Aidan Lister] says $3 million is probably more realistic so I'll try to be conservative,” the 25-year-old university student says.

“You cannot accurately predict what's going to happen with onesies. Last month we sold out many best-selling suits and turned over $250,000, in June we expected 10 per cent growth on May's figures and then posted 54 [per cent]. It's crazy.”

Labib and Lister met at Monash University before discovering the onesie craze during a snow trip to northern Japan in 2008-09. Such was the interest from mates back home they launched Kigu.me in February 2012, built a website in two days and made their first online sale three days later.

“We had 400 on order and before they had even arrived in Australia, we had pre-sales on all of them. It hasn't stopped.”

Kigu.me now designs its own onesies and has a manufacturing partner in China that exclusively produces its suits. It recently secured licences to produce Sesame Street animal character onesies – expect Cookie Monster sightings by Halloween and My Little Pony by Christmas – and by October will launch children's sizes, all of which needed to pass stringent fire safety testing to ensure they were made with low fire danger fabrics.

“It has been quite a challenge” producing flame-resistant children's animals suits with tails and ears, Labib says.

“One of the biggest commercial challenges is we get a lot of others trying to copy us and are constantly on the watch for others trying to use our name. One company recently ripped off words and images from our website for their own 48-hour sale . . . regretfully we had to engage our lawyers.”

Rival supplier Kigu Zoo, also headquartered in Melbourne, was the first local company to import “authentic” Japanese animal onesies and is now the exclusive Australian supplier of SAZAC garments, managing director Alex Hellier says.

SAZAC is widely credited as the original producer of kigurumi animal character suits.

“About half our stock sells online, the other half via national retail partners, and we have probably grown by five times in 2013 since 2012 since the SAZAC deal,” Hellier says.

“We have about 30 Hello Kitty character onesies and they are slowly shrinking adult sizes to fit children. There is definitely longevity in the kids' lines.”

Monkey Jar owner Nova Daniel believes the onesie market has reached saturation and the once red-hot craze has come off the boil.

When she took 350 animal onesies to the Splendour in the Grass music festival in 2010 – offering them for $75 apiece and selling out in three days – she says her Melbourne shop was the only SAZAC stockist in Australia. Now containers of much-cheaper replica import stock are “flooding the market” and she does not expect to see trendsetters wearing onesies in 2014.

“They will be done by the end of the year. We started only selling them to adults as a bold fashion statement and in my years' experience, as soon as large groups of 10-year-olds get hold of a trend, its days are almost certainly numbered.”