An enormous stick insect more than half a metre long and a bizarre lungless frog are among a staggering collection of new species highlighted today to celebrate an agreement to conserve wildlife on the island of Borneo.

Conservationists say the weird and wonderful creatures were discovered thanks to a pioneering deal between three governments to protect and conserve 220,000 square kilometres of lush rainforest on the island.

Some 123 new species have been recorded in the protected region, known as the "Heart of Borneo", since the 2007 agreement.

They include a vivid flame-coloured bronzeback snake that can flare the back of its neck to reveal bright orange colours when threatened, a new bird named the spectacled flowerpecker, and a green and yellow slug with a tail three times the length of its head.

Adam Tomasek, leader of conservation group WWF's Heart of Borneo initiative, said: "As the past three years of independent scientific discovery have proven, new forms of life are constantly being discovered in the Heart of Borneo. If this stretch of irreplaceable rainforest can be conserved for our children, the promise of more discoveries must be a tantalising one for the next generation of researchers to contemplate."

Explorers have visited Borneo for centuries, he said, yet vast tracts of its interior are yet to be biologically explored.

The new species identified on Borneo are highlighted today by a WWF report on the conservation project, which began in 2007 with an agreement between Brunei Darussalam, Indonesia and Malaysia.

The reserve is an "island within an island" the report says, home to 10 species of primate, more than 350 birds, 150 reptiles and amphibians and 10,000 plants that are found nowhere else in the world. The discovery of three new species a month since the agreement offers "ample justification" of the decision to protect the region, it adds.

New species include a giant stick insect, known as Chan's megastick, that measures 57cm long. Discovered in 2008, only three specimens have been found. It is believed to inhabit the high rainforest canopy, which makes it elusive and difficult to study. Borneo has long been known for its monster insects, including giant cockroaches some 10cm long.

A long-tailed slug, dubbed "ninja slug", was discovered on leaves in high-altitude forests, where the creature likes to wrap its long tail around its body while it rests. The slug is part of an unusual invertebrate family that uses chalky "love darts" in courtship. The tiny harpoons pierce and inject hormone into mates, and may increase the chances of reproduction.

Under the 2007 plan, the three governments have committed to enhance management of the protected area, develop eco-tourism and support sustainable resource management.

"Three years on, the Heart of Borneo declaration is proving to be an irreplaceable foundation for conservation and sustainable development by establishing a framework for action to protect Borneo's globally outstanding biodiversity, ecosystem services and livelihoods," Tomasek said. "The discovery of these new species in the Heart of Borneo underlines the incredible diversity of this remarkable area."