Proverbs 30:15

He had spoken before of those that devoured the poor (Pro 30:14), and had spoken of them last, as the worst of all the four generations there mentioned; now here he speaks of their insatiableness in doing this. The temper that puts them upon it is made up of cruelty and covetousness. Now those are two daughters of the horse-leech, its genuine offspring, that still cry, "Give, give, give more blood, give more money;" for the bloody are still blood-thirsty; being drunk with blood, they add thirst to their drunkenness, and will seek it yet again. Those also that love silver shall never be satisfied with silver. Thus, while from these two principles they are devouring the poor, they are continually uneasy to themselves, as David's enemies, Psa 59:14, Psa 59:15. Now, for the further illustration of this, I. He specifies four other things which are insatiable, to which those devourers are compared, which say not, It is enough, or It is wealth. Those are never rich that are always coveting. Now these four things that are always craving are, 1. The grave, into which multitudes fall, and yet still more will fall, and it swallows them all up, and returns none, Hell and destruction are never full, Pro 27:20. When it comes to our turn we shall find the grave ready for us, Job 17:1. 2. The barren womb, which is impatient of its affliction in being barren, and cries, as Rachel did, Give me children. 3.

— Matthew Henry's Concise Commentary (public domain)

horse leech--supposed by some to be the vampire (a fabulous creature), as being literally insatiable; but the other subjects mentioned must be taken as this, comparatively insatiable. The use of a fabulous creature agreeably to popular notions is not inconsistent with inspiration. There are three . . . yea, four--(Compare Pro 6:16).

— Jamieson-Fausset-Brown Commentary (public domain)