What is the equivalent in Chinese for the English word "horny"?
Horny
4. (Slang: Vulgar): lustful; sexually excited.
發情
be horny; libidinous
Examples:
He is horny - 他在發情
Horny woman -  發情的女人
Horny man - 發情的男人

To be subtle, you can use 亢奮

亢奮
very excited
Examples:
He is horny - 他亢奮起來