Genopunks and The Geneticist's Beastiary
At first glance, multicellular organisms would seem like the least effective means for trolling a population. Not only are they visible to the naked eye, but they cast an infrared signature is a mile wide. Synthetics are quicker to build, cheaper to maintain, and more easily modified once fully grown. Knowing this, why bother with plants and animals?
Several reasons, in fact. For one, autonomous replication. While synths may eventually outlast an animal, they can't effectively reproduce fine components except through nanotech, and self-replicating nanotech doesn't exist yet. They could carry special fabbers on their bodies, but this puts a floor under their size; not even protean hives scale below that of a large insect. Reliance on nanoassembly also leaves these robots dangerously vulnerable to EMP. True self-replicating machines are beyond the understanding of all but transhumanity's worst nightmares.
Organic molecules, on the other hand, are a proven and reliable alternative. Replication from a single cell on up is possible. Wombs and eggs need not be very large. While circuits, sensors and batteries need exotic elements that can only be scavenged from similar electronics, organic compounds are often cheaper and more ubiquitous in most environments. Where food is plentiful they can replicate exponentially. A small investment of time at a sequencer can wreck immense returns in damage, as several Consortium ships found out when they were seeded with aerogel-eating weevils after the first Battle of Locus, and lost heat just long enough to freeze a few dozen biomorphs.
Complex organisms can also mirror the threat vectors of larger robots. Basic biomods augment the body's immunity to most bacteria and viruses, and nanomedicine can take care of the rest. So while engineered pathogens are still a threat against the unvaccinated, they are not in intractible one. Neither are teeth and claws, of course, but medichines can only heal their damage, not eliminate the source. Thus multicellular life still has a place in the genehackers' arsenal.
It should be noted that not all genopunks are the malicious imaginings of xenocidal terrorists and ironic mercurials. Some are just harmless pranks that will irritate a population for one breeding season before hab security scrubs them back to the Pre-Cambrian. Tenacious species will often hide quietely in the slums or sensor blind spots of larger habitats, their presence reduced to a tolerable nuissance not worth the diminished returns of a more thorough eradication campaign, which will only provoke the trolls into releasing an upgraded batch.
Reactions can vary widely between factions. Sometimes the presence of multiple genopunks keeps them all in equilibrium, and forward-thinking societies employ ecologists to help with these calculations, even inserting their own safe organisms into the equation. This is the route taken by many Lunar-Lagrange habitats, particularly those with an Indian cultural majority. Some Anarchists take it even further, their adherence to animal liberation spurring research into zoosemiotics in an attempt to communicate with the creatures. By stark contast, Jovian Biosecurity will torch any unlicensed transgenics on site.
Extropian habs regard population control as a problem of individual proprietors and their contractors. Sometimes these firms manage to collaborate on thoroughgoing eradication schemes, but more commonly pests just get pushed only someone else's property, prompting an endless torrent of litigation. Scum fleets tend to tolerate even the most dangerous creatures, as watching a swarm cat fight an Uberratte makes life more interesting. Customs authorities around the system issue especially loud groans whenever one of their shuttles makes planetfall.
It's not even the case that biologicals are always harder to hide than synthetics. They don't set off metal detectors, and chem sniffers are useless when they share the same basic components as every biomorph. Without a deliberate x-ray scan it's especially difficult to detect organisms hidden inside another, which makes parasites particularly effective against targets without medichines.
Genehackers can further conceal their progenies with a bit of creative with bioware. Chameleon Skin is a common choice, and they can augment it with heat sinks like a Chameleon Cloak. Chitinous radar-absorbant exoskeletons are another option; the giant Stealth Urchin even attains full invisibility with transparent metamaterials. Still, the easiest method by far is to remain small enough for radar's low-resolution to miss you.
Threats and pranks aside, most genopunks are actually harmless and well intentioned. Like smart pets, people create them for aesthetic reasons, companionship, or just because they can. Many roam freely on habitats, fed by residents or the native plant life.
Over the years, numerous species have been released into ecosystems of Mars, to mingle with the more purpose-built terraformers. Even the dangerous ones are largely left to their devices, too firmly rooted to be worth eradicating just to make life a little safer for rednecks no one cares about. Besides, they breath, and more biomass means more carbon dioxide to heat the planet. Carbon never does go out of style.
Example: Somatek Geijigeiji
An experimental smart pet gone waaaay wrong. Somatek attempted this years ago, basing it on the large scutigera centipedes once kept as pets by some Japanese people. The idea was to create a geijigei that looked cute and could produce rudimentary speech. Unfortunately, Somatek did something to cross one of the team geneticists, and sometime after the final trials but before the first blueprint release they managed to slip a replacement in without anyone knowing. At first, this new design was indistinguishable from the original. Once the geijigei reached maturity, however, things took a turn for the sanity shattering.
This monstrous geijigei is approximately a half meter long, with exactly 100 legs attached to a black chitinous exoskeleton, and equipped with dense network of fine hairs enabling it to run on air for short bursts. Its face is like that of the the original centipede, save for a grotesque human-like mouth. Through a heavily modified set of the organs of Tömösváry, it can produce sounds of variable range and amplitude. Lacking a cybercortex it is incapable of understanding what sounds it produces.
The geijigei hunts by attempting to mimic the sounds of its prey, repeating single words or phrases over and over. "Please help me" is a common choice. Once the prey has approached close enough, the geijigeiji launches into a full run, travelling at 50 meters per round, screaming horribly to disorient it. Once it has overtaken the prey, it injects a powerful cocktail of neurotoxins via its forcipules. After which it feeds.
Naturally, once this problem became apparent Somatek immediately issued a recall and built itself a bunker out of lawyers and PR consultants. A thorough extermination ensued on the habitats effected, but biosecurity teams could not account for all the purchased specimens. Contrary to the standard reproductive restrictions on smart pets, female geijigei can lay hundreds of eggs. Specimens survive by hiding in out of the way places like Lunar and Martian slums, the sanitary sewers of the same, or on the less stringently maintained habitats and ships of scum fleets. Also in nightmares.
Neo-Chupacabra - A scaley, spiny predator in the shape of a large dog, harvesting nourishment from transgenic livestock and other luxury "real foods", the "reindeer" of Titan's hulder population, and unlucky biomorphs by extracting fluids and liquefied organs. The Titanian variant's harvesting bypasses the need to replenish the chemicals needed to survive the cryonic environment.
Moledog - A dog with tentacled heads and no sight living as packs in the Martian cave systems.
Uberatte - A giant rat with powerful teeth, able to chew through metal and potentially even cause hull breached. Variants come equipped with chameleon skin and heat sinks.