Learn English – What’s another word for Guinea Pig, i.e. when you call someone a “test dumthe”

Guinea pigs might not be the most common experimental subjects today, but we have to look at how common they were when the term was first coined, which was many decades ago, as Etymonline points out:

In the extended sense of "one subjected to an experiment" it is first recorded 1920, because they were commonly used in vivisection experiments.

Wikipedia seems to be backing that up:

Biological experimentation on guinea pigs has been carried out since the 17th century. The animals were frequently used as a model organism in the 19th and 20th centuries, resulting in the epithet "guinea pig" for a test subject, but have since been largely replaced by other rodents such as mice and rats.

Looking further, I found this essay on the History of the Guinea Pig:

Guinea pigs have also played a very important role in the field of medical research, and although most pet lovers today may cringe at the thought of animals being used in laboratory research, the historic and important contribution of guinea pigs to science cannot be overlooked. Guinea pigs are often used as a metaphor for any subject of scientific experimentation (e.g. "human guinea pigs"), and this idea persists even though guinea pigs are no longer commonly used as modern experimental animals, as rats and mice (which breed quicker) have replaced them. In 1890 the antitoxin for diphtheria was discovered using guinea pigs in the research, and as a result the lives of millions of children have been saved. The guinea pig's wide variety of hair types and colours have also made them a prime choice for studies of genetics and heredity. During the 20th century a special strain of smooth-coated white guinea pigs called Duncan-Hartley were bred specifically for laboratory work.

Emil Adolf von Behring actually won the (first ever) Nobel Prize in Physiology or Medicine in 1901 for developing a serum therapy against diphtheria. He used guinea pigs in his experiments, though later the serum was extracted from sheep and then horses.

Someone suffering from tall poppy syndrome.

Tall poppy syndrome (TPS) is a pejorative term primarily used in the UK, Canada, Australia, New Zealand and other Anglosphere nations to describe a social phenomenon in which people of genuine merit are resented, attacked, cut down, or criticised because their talents or achievements elevate them above or distinguish them from their peers.

eg.

Sue was suffering from tall poppy syndrome. After her cousin Kathy landed a high paying modeling job in New York, she complained loudly at the family gathering about how 'Kathy thinks she's so much better than us now that she's working in New York. Pity it's not going to make her an any nicer person'.