The hyena in its solitude is like a lion. الذِّيخُ فِي خَلْوتِهِ مِثْلُ الأَسَدِ‏.‏

 (The hyena: the male of the hyena.) This proverb is used to describe someone who claims to be capable of something when alone but cannot achieve it when challenged in a group; it is similar to the saying, "Every man who runs alone is pleased."
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Some of the closest English proverbs and expressions to this proverb are:
“Every dog ​​is a lion at home.”
Meaning: Every dog ​​becomes a lion in its own den.
Said of someone who is brave in a safe place or when alone.
“A lion at home, a mouse abroad.”
Meaning: A lion at home, but a mouse outside.
This is very close in meaning: Someone who pretends to be brave when alone but is helpless when confronted.
“Big talk, little action.”
Meaning: A lot of talk, little action.
Said of someone who exaggerates but doesn't stand up to the test.
“All bark and no bite.”
Meaning: All bark and no bite.
This is one of the most common sayings of someone who threatens and pretends to be strong without any real ability.
And the closest to the spirit of the Arabic proverb is often:
“A lion at home, a mouse abroad.”
“A lion in his house, a mouse outside.”
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الذِّيخ‏:‏ الذكَرُ من الضِّباع‏.‏
يضرب لمن يَدَّعي منفرداً ما يعجز عنه إذا طُولب به في الجمع، وهذا مثل قولهم ‏"‏كُلُّ مُجْرٍ في الخَلاَء يُسَرُّ‏"