These are usually called a "fifth wheel" or "gooseneck" trailer.
The type of tractor trailer that can hold the heaviest loads is a Semi-Trailer truck. This is the heaviest truck that is allowed on the roads.
An 18 wheeler is a semi-truck and trailer. A truck, and trailer, has 18 wheels, including 10 on the semi, and 8 more on the trailer. 18 Wheelers are responsible for the majority of products the user buys everyday.
Permanent Trailer (PTI)
A large semi-tractor that pulls it just like a trailer.
5W30
It really depends on what size trailer you are interested, if it is something that can be towed behind a pickup truck, or are you looking for a large trailer for a semi truck, also whether you are looking for an open trailer or an enclosed trailer. Most Metropolitan areas might have local trailer manufacturers, if looking online, I would recommend Big Tex trailers, www.bigtextrailers.com
You need a commerical auotmobile policy to covere the vehicle and trailer. Additionally, you will need a General Liability policy that coveres "other peoples goods and property" to cover the horses.
Green
10W30 Gastrol GTX works real good.
Either universal green or red ELC (extended life coolant), depending on what engine it has.
Utility trailers, landscaping trailers, camper trailers, all kinds of trailers as long as the don't exceed the trucks maximum towing weight. The maximum towing weight depends on the model (2 wheel or 4 wheel drive), engine, wheel base and GVW of the truck. Rule of thumb is that the trailer should not exceed the weight of the truck.
Book says : SAE 75W90 synthetic gear oil only.