It doesn't look like any Horizon I'm familiar with.  Depending on the year there should be a HIN# at the right rear corner area.  The first three letters will indicate the builder. 

From the pics provided it doesn't look like a "flat bottom" and calling it a "flat bottom" or "flatty" will cause the true "flat bottom" people to turn into absolute whinny little bitches 

I'd like to see some more pics, it is a unique hull design and I would like to know more about it.

P.S.  Not a fan of the mud bog truck headers, but it looks like a great start to a fun boat.

Welcome to SCJB