a wolf in sheep's clothing
US /ə wʊlf ɪn ʃiːps ˈkloʊðɪŋ/
UK /ə wʊlf ɪn ʃiːps ˈkloʊðɪŋ/

1.
un lobo con piel de cordero
a person who appears friendly or harmless but is actually dangerous or malicious
:
•
Be careful of him; he's a wolf in sheep's clothing.
Ten cuidado con él; es un lobo con piel de cordero.
•
The new manager seemed nice at first, but he turned out to be a wolf in sheep's clothing.
El nuevo gerente parecía amable al principio, pero resultó ser un lobo con piel de cordero.