He spent 30 years at Glasgow Zoo in Calderpark, working with lions and tigers, bears and rhinos, reptiles and birds.
So when the zoo closed in 2003, it was no surprise he went to work for the Scottish Society for the Prevention of Cruelty to Animals (Scottish SPCA). He has now been an inspector for the animal charity for six years.
Covering the city centre and north Glasgow, Billy, 60, from Uddingston, rescues animals from abuse and neglect, as well as those that have been injured.
A typical day can see him and the other inspectors deal with dozens of cases that have been reported by the public via the Scottish SPCA's Animal Helpline.
He said: "I start at 7.30am and am supposed to finish at 5.30pm, but sometimes you can be on for much longer.
"If you are on-call you can be working until 10pm, it can be quite a long day.
"The other night a road accident came in 10 minutes before the end of my shift but I couldn't just leave it.
"It was a roe deer in Bishopbriggs, it was injured but still quite mobile so it was difficult to catch."
Working for an animal charity may sound like a far cry from the zoo, but sometimes there isn't too much of a difference in the clientele.
He said: "You can get a lot of exotic pets. On a regular basis we get snakes and lizards that people have abandoned when they move house.
"I found a scorpion in a tank once in a flat in Castlemilk.
"The place had been burgled and the boy who lived there had fled, but I don't think the thieves wanted to take the scorpion.
"Two years ago I rescued two dogs and a cat from a house on the South Side and when I was checking the property for more animals, I found two large anaconda snakes in a tank made out of a wardrobe and plastic."
As well as walking on the wildside, Billy also deals with domestic animals, such as cats, dogs and farm animals.
"I've been out to farms where the animals are suffering neglect," he says. "Once I went out to see some donkeys with overgrown hooves.
"Sometimes we attend to overweight dogs. Some owners think they're doing the right thing feeding their dog up, but it will suffer the same health problems as a person who's clinically obese, and can be at risk of diabetes and heart and liver problems."
In 2011, the Scottish SPCA Glasgow Animal Rescue and Rehoming Centre, based at Cardonald, cared for more than 2000 animals, half of that number being dogs, 605 cats, 77 rabbits and more than 200 smaller animals, such as guinea pigs, rats, ferrets and birds.
SSPCA staff across the west of Scotland attended nearly 16,000 call outs last year – almost 300 a week, which included investigations, rescues and abandonments. Across Scotland the figure rose to almost 50,000 in 2011.
"The saddest part is if the animal dies or needs to be put to sleep," says Billy.
"Most are rehomed, or released back into the wild.
"The most common cases are neglect, but we're seeing abandonments increasing.
"I think this is a combination of a few things, maybe people can't afford the pet or they may go away overnight and haven't made adequate provision for their animal.
"Your job as an inspector is to prevent cruelty to animals and promote kindness and respect in their treatment.
"Most of the time it's about educating the pet owner, sometimes you get people who kick off but most are happy to take advice."
There is a process with dealing with owners who have been deemed liable for neglect, starting with v erbal advice, then a welfare notice, followed by a caution, before reporting owners to the procurator fiscal, which can lead to prosecution under the Animal Health and Welfare Act 2006 in severe cases.
"I went to a job in Drumchapel a few months ago and found a dog in the back court of a block of tenements," said Billy.
"It was dead and there was nothing to it, just skin and bones, it was horrific. Our vet said it was the worst case of starvation he had seen.
"We traced the owner through the media and got a result just the other week, they got a 10 year ban on keeping animals and 18 months' community service."
Although certain aspects of the job can be harrowing, there's nothing else Billy would rather do.
"Nature is nature and everything deserves a chance," he says. "I love Glasgow, everyone you meet wants to help you, even the people who have done something wrong take the advice and they want to do the right thing for their animal."
