ҷосус

Дар байни онҳо ҷосус буд.
There was a spy among them.
Бо мақсади забт кардани ин сарзамин онҳо ҷосусони худро мефиристоданд.
With the intention of conquering this land they sent their spies.

Bonus:
ҷосусӣ: espionage
ҷосисӣ кардан: to spy

Animate objects (people, animals) can take the plural ending -он. This is especially seen in written work. Colloquially and informally, you can still use -ҳо suffix if you prefer.
ҷосусон, ҷосусҳо: spies