but how come when with horse, you'd say horses?

English is a funny language..

Plural of mouse is mice..

But plural of house is not hice; it's houses..

However, plural of louse is lice

we know the opposite of pro is con, but why is the opposite of progress....

nevermind, that one actually works.

The opposite of 'pro' is 'con' only as in 'contra'. The 'con' root in 'congress' is from Latin 'com' which means together, not 'contra' which means against.