The story
From the economics of horse ownership, in which an aging horse of declining value continues to require feeding and stabling. Possibly influenced by the proverb don't look a gift horse in the mouth.
Etymology from Wiktionary, CC BY-SA 4.0.
gift horse
Today it means: An apparent gift, that has substantial associated costs or drawbacks, especially a gift that does not fit perfectly with the recipient's wants or needs. Full definition →