“Stock Trailer” means a lot of different things and it’s important to understand what you are getting and not getting.
In general, it is a catchall term to mean a trailer with open, slatted sides.
However, within that consider:
- axle rating, to know how much weight it can carry
- height and width
- look for sharp horse-unfriendly edges
- suspension - torsion axles have a much nicer ride than leaf-spring
- can have dividers or not
- can have tie points or not
- what keeps the animals in - are the latches able to take a horse sitting on it
There are many stock trailers I’d happily put my horse in and many I never would and some I’d use in a pinch but not buy. It’s kind of like “sedan” which could be a Yugo or it could be a Mercedes. 
Not only are some stock trailers older but many people are… maybe a bit less concerned about individual livestock animals than we are with our horses. So for example, I know of a local incident where a pig fell out of an older stock trailer while it was going down the road.
Horses have a higher center of gravity than other livestock animals and are more mobile and more reactive. So, a trailer that easily handles 10 lambs might still not work for a horse.
In general, a typical stock trailer is going to be shorter in height and narrower than a newish horse trailer.
You can buy a “horse” trailer with removable dividers and with or without a ramp, and even with stock-type sides. You can buy a “stock” type trailer with a dressing room, with fancy dividers, a ramp, top of the line suspension, extra doors, etc.
I use my horse trailer sans dividers to move all kinds of things. (The ramp is really nice for appliances…)
I prefer a horse trailer to have a butt bar restraining the horse inside the doors, rather than relying on the door to hold the horse in. First, the latch may bend or fail; second, if it bends, it may be hard to get the door open.
If a trailer is cheap, it was made cheaper in some way. It’s good to know how and why. Good basic transportation is all you need, but you want the corners to be cut on fancy things like extra doors and not on structural integrity, finish, suspension, or weight ratings.