Seems like I’m on a monster making kick here. My present the Rat Roach for Swords & Wizardry.
The Rat Roach
HD: 1d4 HP
AC: 9 
Attacks: Bite (1 to 2 Damage)
Saving Throw: 18
Special: Immune to poison, Magic Resistance, Plague Carrier, Hard to Kill.
Challenge Level/XP: 2/30
Many wizards don’t keep the neatest of laboratories and when an accident happens there may be unforeseen consequences. Such is the Rat-Roach. An alchemical experiment went awry created this odd chimera. Part rat. Part Cockroach. Individually, they are more of a nuisance than a threat. But in large numbers, they can cause plagues or worse.
Rat Roaches are immune to poison and have 30% Magic Resistance. When a character is bitten by a Rat Roach, the character must make a Save versus disease at +2 or contract the plague. The first time a Rat Roach is “killed”, it makes a Saving Throw. If successful, the Rat Roach is only severely injured and attempts to crawl away.