The horseleach hath two {h} daughters, crying, Give, give. There are three things that are never satisfied, yea, four things say not, It is enough:

(h) The leach has two forks in her tongue, which here he calls her two daughters, by which she sucks the blood, and is never satisfied: even so, the covetous extortioners are insatiable.

Wesley's Notes

30:15 The horse - leach - An insatiable creature, sucking blood 'till it is ready to burst. Two daughters - The following things resemble the horse - leach in its insatiableness; nothing being more ordinary than to call those persons or things the sons or daughters of those whose examples they imitate. Three - Though he begins with two, yet he proceeds from thence to three, and four, all which are said to be the daughters of the horse - leach.

King James Translators' Notes

It is...: Heb. Wealth

Jamieson-Fausset-Brown Bible Commentary

15, 16. 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 Pr 6:16).

Matthew Henry's Concise Commentary

30:10 Slander not a servant to his master, accuse him not in small matters, to make mischief. 11-14. In every age there are monsters of ingratitude who ill-treat their parents. Many persuade themselves they are holy persons, whose hearts are full of sin, and who practise secret wickedness. There are others whose lofty pride is manifest. There have also been cruel monsters in every age. 15-17. Cruelty and covetousness are two daughters of the horseleech, that still cry, Give, give, and they are continually uneasy to themselves. Four things never are satisfied, to which these devourers are compared. Those are never rich that are always coveting. And many who have come to a bad end, have owned that their wicked courses began by despising their parents' authority. 18-20. Four things cannot be fully known. The kingdom of nature is full of marvels. The fourth is a mystery of iniquity; the cursed arts by which a vile seducer gains the affections of a female; and the arts which a vile woman uses to conceal her wickedness. 21-23 Four sorts of persons are very troublesome. Men of low origin and base spirit, who, getting authority, become tyrants. Foolish and violent men indulging in excesses. A woman of a contentious spirit and vicious habits. A servant who has obtained undue influence. Let those whom Providence has advanced from low beginnings, carefully watch against that sin which most easily besets them.