Having searched through what RM & ME I could I found no darkling creature.
One 'solution' I've come up with involves the use of Larva from AD&D 1st ed. as a "very minor demon" familiar.
Although it may break size rules, it is a fairly hapless creature and may form a challenge to carry around.
Using the original C&T section on converting AD&D monsters into RM here is my conversion of the Larva from MM 1st ed into RM2.
Larva |
Level 1 |
Base Rate 25' (0) |
Max Pace: 1.5 |
MS: SL |
AQ: VS |
Size M |
Hits 25A |
AT 3(0) |
Attack: 18Ti |
The familiar can be 'upgraded' as spell states demon can "advance in experience."
This involves HR use of spells and rituals to create a Quasit also from MM 1st ed.