Pull your horns in
Restrain one's ardour; lower one's ambitions.
You might think that warning someone who had overreached themselves in some dynamic enterprise to 'pull their horns in' was a reference to some pugnacious horned creature, maybe a bull or a rhinoceros. In fact, the creature in question is the unassuming garden snail. The retractable tentacles on which a snail's eyes are located are known as horns, and these are pulled in whenever the mollusc is threatened.
Likewise, the date of origin of what has the sound of a fairly recent colloquialism, is not what might be expected. Snails were spoken of as 'pulling their horns in' as early as the 14th century. An example of that is found in this extract from the Anglo-Norman romantic poem, Richard, Coer De Lyon:
And gunne to drawen in her hornes,
As a snayl among the thornes
That citation uses the common alternative form 'draw in one's horns'. Other less common varieties are 'shrink/pluck in one's horns'.
An early use of 'pulling in one's horns' as a figurative phrase, with no snails in sight, comes in the 16th century religious diatribe Hay any Work for Cooper, 1589:
Mark how I haue made the bishops to pull in their hornes.
[Note: 'Hay any work for cooper' was a medieval street cry of the coopering (barrel making/repairing) trade]