Menderbug

Behaviour and Tactics
Menderbug has an extremely low chance to spawn at the entrance of Forgotten Crossroads right below Dirtmouth. To spawn, the following conditions must be met: Upon seeing The Knight, the Menderbug will flee and quickly leave the room.
 * The sign at the entrance must be destroyed
 * The player must leave and re-enter the room

One of the easiest ways to catch it is in late game after acquiring either Kingsoul and/or Salubra's Blessing to refill SOUL easily. Enter Forgotten Crossroads from Dirtmouth by using Desolate Dive. This will destroy the sign upon landing and, if it has spawned, will also kill the Menderbug. It can also be spawned by leaving the room to the left or right within Forgotten Crossroads but that takes significantly more time and it's easier to miss the chance to kill it before it flees.

Trivia
Починжук Insecto_reparador
 * Menderbug is actually not needed to 100% the game.
 * Oddly, despite being labeled as an enemy, Menderbug doesn't seem to be infected.