Charles H. Spurgeon on Psalms 66:15

In the burnt offerings, we see his approach to the altar with the common and general sacrifice; and next, in his paying vows, we see he has brought his peace offerings with him. Again, therefore, he says at the altar: I will offer unto thee burnt sacrifices of fatlings (Ps 66:15). This is the general offering, brought from the best of his flock and herd. Then follow the peace offerings: With the incense (trjq, fuming smoke) of rams; I will offer bullocks with goats. Selah. Having brought his offerings, he is in no haste to depart, notwithstanding; for his heart is full. Ere, therefore, he leaves the sanctuary, he utters the language of a soul at peace with God: Ps 66:16-20. This, truly, is one whom the very God of peace has sanctified, and whose whole spirit, and body, and soul he will preserve blameless unto the coming of the Lord Jesus Christ. 1Th 5:23. Andrew A. Bonar.

Verses 13-15. He tells what were the vows he promised in his troubles, and says he promised the richest sacrifice of cattle that could be made according to the law. These were three —rams, cows, and goats. Rams included lambs; cows included heifers; and goats, kids. Robert Bellarmine.