When an ox enters a palace, it doesn’t become a king but the palace turns into a barn.

[Öküz saraya çıkınca kral olmaz. Ama saray ahır olur.]

(Other Authors and Sources)
Circassian proverb

This "Turkish" (properly Circassian or Adyghe) proverb (source) can be found with a variety of forms and choice of livestock. It was famously used by journalist Sedef Kabaş in early 2022:

A bull does not become king just by entering the palace, but the palace becomes a barn.

Kabas was jailed for supposedly insulting Turkish President Erdoğan with the reference.

Based on Kabas' usage, it was reworked on 23 January 2022 by Elizabeth Bangs, a British academic consultant, with British Prime Minister Boris Johnson in mind, and posted on Twitter into a more viral form:

When a clown moves into a palace he does not become a king. The palace becomes a circus.

The clown version is now widely misattributed as a Turkish proverb.

More notes: