English Idioms and Phrases: Don't look a gift horse in the mouth
When someone tells you 'not to look a gift horse in the mouth' it is advice not to doubt or criticize a potentially good situation or thing that has been offered to you.
"You should take that new job, don't look a gift horse in the mouth"
Another English idiom that is about a horse and this one is closely related to 'long in the tooth' in its origin. (Click link)
The horses gums recede with age and therefore the higher the gums the older the horse. If you received a horse as a present or gift it was seen as rude to examine the horses mouth to find out how old it was rather than be grateful for the gift itself therefore 'don't look a gift horse in the mouth'
Alternatives or Synonyms
Don't be ungrateful, Don't criticize