The panther chameleon was found on Monday by a dog walker in the wooded area at Marl Park.
It had to be put down after X-rays showed all of its legs were broken and it had a deformed spine.
RSPCA Cymru said it was an "extremely sad example of an abandoned and neglected exotic pet".
Inspector Selina Chan said: "It is a possibility that the owners took on this animal but were unable to provide the care he needs and decided to release him to the wild.
"We are urging potential owners of exotic animals to thoroughly research what is required in the care of the particular species before taking one on.
"Potential owners need to make sure they can give their animal the environment it needs and they have the facilities, time, financial means and long-term commitment to maintain a good standard of care, as required under the Animal Welfare Act 2006."
She added it was illegal to release non-native species into the wild.

He succeeds Laurent Fabius, 69, who announced on Wednesday he was stepping down as part of a cabinet reshuffle.
Mr Ayrault is, like Mr Fabius, a former prime minister, leading the government from 2012-14.
The leader of the Greens, Emmanuelle Cosse, has been named housing minister, and two other Green politicians have been given junior posts.
Segolene Royal - a former presidential candidate and Mr Hollande's ex-partner - has been given an expanded portfolio, becoming minister for environment, energy and the oceans, the Elysee Palace said.
Mr Fabius was seen as the architect of an interventionist foreign policy that saw French troops battle militants in Mali and take part in coalition air strikes against the so-called Islamic State group in Syria and Iraq in recent years.
Mr Fabius has been named to head France's Constitutional Council, which ensures bills comply with the constitution.

A television documentary and two radio productions took the top prizes in their categories for the BBC.
Peter Johnston, BBC Northern Ireland's director, said the awards were "a great acknowledgement of the creativity and dedication of our production teams".
The three-day festival took place in Dungarvan in County Waterford this week.
BBC One NI's True North: Keepin' Er Country took the main accolade in the factual entertainment category.
Made for the BBC by the Alley Cats production company, it told the story of country music fans with a passion for dancing.
BBC Radio Ulster's Saturday morning programme The John Toal Show won the best radio magazine show award.
And BBC Radio Ulster also took the main award in the radio documentary section for Lacrimosa, produced by Conor Garrett.
The programme, broadcast last summer, was a personal look at why presenter Proinsias O'Coinn found it difficult to cry.
It has also been selected for the 2016 International Features Conference.

The BBC's Ian Pannell said the jihadists were putting up a hard fight.
BBC producers Joan Soley and Peter Emmerson are also embedded with the Iraqi troops.
BBC Arabic's Feras Kilani caught in crossfire.
This picture from Feras Kilani shows the destruction of a neighbourhood on the outskirts of the city.
There are thought to be up to 1.5 million civilians in Mosul, trapped between IS and Iraqi forces.
The team were with the Counter-Terrorism Service (CTS) when it retook Bazwaya, about 3km (2 miles) east of Mosul, on Monday.

After his opening 66, two immediate birdies moved the 37-year-old to a share of the lead but six bogeys in windy conditions saw him losing ground.
A record nine players share the halfway lead on eight under.
These include first-day leader Welshman Bradley Dredge (72) and English Ryder Cup player Andy Sullivan (68).
Thailand's Kiradech Aphibarnrat, South African duo Thomas Aiken and Jaco Van Zyl, Spanish pair Jorge Campillo and Nacho Elvira, Finland's Mikko Korhonen and South Korea's Jeunghun Wang are also in the leading nine-strong group.
Ireland's Paul Dunne is sharing 10th place, a stroke off the pace, after adding a 70 to his opening 67.
2010 US Open champion McDowell has dropped to 89th in the world rankings, and as it stands, is not in the fields for either the WGC World Match Play in March or the Masters a month later.
Owners of exotic animals have been urged to do research before having them as pets after a seriously neglected chameleon was found in Cardiff Bay.
[NEXT_CONCEPT]
French President Francois Hollande has named Jean-Marc Ayrault as his new foreign minister.
[NEXT_CONCEPT]
BBC Northern Ireland programmes have won three awards at the Celtic Media Festival in the Republic of Ireland.
[NEXT_CONCEPT]
As Iraqi special forces enter the city of Mosul for the first time since the start of the campaign to retake it from so-called Islamic State, a BBC team embedded with the troops is tweeting from the front line as the battle unfolds.
[NEXT_CONCEPT]
Graeme McDowell dropped five off the pace at the Qatar Masters after struggling to a three-over-par 75 in Friday's second round in Doha.