It is like the ibex, rarely seen.--إنما هو كبارح الأروى قليلاً ما يرى

 This is because the ibex inhabits the mountains, and people hardly ever see them, whether coming or going, except perhaps once in a lifetime. This proverb is used to describe someone who shows kindness only occasionally, and the phrase "it is like the ibex" is a metaphor for the generosity and giving that is the subject of the proverb.

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The closest English equivalents are:

Once in a blue moon
“Very rarely, if ever.”

This is the closest English proverb to describe extreme rarity.

Few and far between
“Rare and infrequent.”

This is very suitable for describing a small act of kindness.

Rare as hen’s teeth
“As rare as hen’s teeth.”

A well-known English expression for something extremely rare.

His kindness is few and far between
“His kindness is rare and infrequent.”

The closest literary translation of the proverb is:

His kindness appears only once in a blue moon.

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وذلك، أن الأروى مساكنها الجبال فلا يكاد الناس يرونها سانحة ولا بارحة إلا في الدهر مرة. يضرب، لمن يرى منه الإحسان في الأحايين، وقوله هو كناية عما يبذل ويعطى هذا الذي يضرب به المثل.