John Gill Commentary Psalms 136:19

"Sihon king of the Amorites; For his lovingkindness [endureth] forever;" — Psalms 136:19 (ASV)

Sihon king of the Amorites
A strong and warlike people, and their king a great and mighty one; see (Psalms 135:11) ;

for his mercy [endureth] for ever ;
since this king would not suffer Israel to go through his borders, but came out and fought against them; and had not the Lord smote him, they must have fallen a prey into his hands, (Numbers 21:23) .