I knew the Australian funnel-web spiders were bad, but I did not know that their venom acted so fast!
Envenomation symptoms observed following bites by these spiders are
very similar. The bite is initially very painful, due to the size of
the fangs penetrating the skin.
Early symptoms of systemic envenomation include goose bumps, sweating,
tingling around the mouth and tongue, twitching (initially facial and
intercostal), salivation, watery eyes, elevated heart rate, and
elevated blood pressure. As systemic envenomation progresses, symptoms
include nausea, vomiting, shortness of breath (caused by airway
obstruction), agitation, confusion, writhing, grimacing, muscle
spasms, pulmonary oedema (of neurogenic or hypertensive origin),
metabolic acidosis, and extreme hypertension. The final stages of
severe envenomation include dilation of the pupils (often fixed),
uncontrolled generalised muscle twitching, unconsciousness, elevated
intracranial pressure, and death. Death generally is a result of
progressive hypotension or possibly elevated intracranial pressure
consequent on cerebral oedema
The onset of severe envenomation can be rapid. In one prospective
study, the median time to onset of envenomation was 28 minutes, with
only two cases having onset after 2 hours (both had pressure
immobilisation bandages applied).
Funnel web males apparently wander around certain times of the year, looking for mates. That is how they run afoul of humans. In your world there is a similar spider (or some other small invertebrate - scorpion might be good) and these things turn up in the prisoner's cell from time to time. Your prisoner knows what they are and after killing a couple, decides to keep one as a pet, feeding it lice from his own body. When the target shows up he surreptitiously flips the creature onto him. The spider seeks refuge in the target's clothes and bites him a minute or two later.
This seems to me more plausible than some ingested poison. If I am an occasional visitor I am not going to have a prisoner make me an omelette or give me a tattoo. I might come close enough to get a spider flicked on me.
If these things live in the prison the guards will probably know what is going on when the target starts gets sick. But an aristocratic target might not deign to tell lowly guards he is feeling poorly, and instead consult with his own entourage or wait to get help from a healer appropriate for his class and station. That would make good narrative, as one of the guards suspects the target has been bitten but then is ignored / dismissed.