If I may make a suggestion to the OP...?
Stirges. Stirges, stirges, stirges, stirges!
You get:
Lots of 'em (no. appearing: 3-30!), they have low hits (1+1HD) but attack as 4HD monsters.
Part of a flock landed on the party's mighty 8th level cleric at Sunday's game and had him down half his...