I am not familiar with the use of equine as a noun. As opposed to bovine, et al., which have become sort of generic terms for all members of a genus, it's an adjective. Doesn't mean you can't use it as a noun, just that I've never heard or read it. What does it mean for you? Another word for horse?

As for the plural, if I used equine as a noun, I'd simply add an -s to form its plural in line with other nouns in English. Or do you see these nouns ending in -ine as mass nouns?

Ceci n'est pas un seing.