Here's the situation:

I am moving and need to pick up horses from 2 different locations after I am set up at my new place near Portland, Oregon. I am a very experienced truck and trailer driver but don't currently own my own. I can use my friend's 16 foot bumper pull but I need a truck. I'd strongly prefer a dually 3/4 ton or 1 ton, obviously has to be a V-8 but probably does not have to be a 454 as I'll only be pulling 2 at a time.

Is there anybody who leases out towing trucks that are all set up to haul a horse trailer? I basically need it for two weeks and the mileage needs to be unlimited. I am willing to pay whatever that would cost and for any necessary additional insurance, but is there anybody on the West Coast who will do that?

All my horses are seniors and I am not comfortable having a professional company haul them, put them with other horses, zig zag all around the route, etc. I definitely want to do it myself. If I have to buy a truck, I will, but since I only need it for 2 weeks, if there's any way of avoiding that, I'd love to know about it.

You might consider U-haul. Their trucks are used to move household goods. I have seen several of them pulling a trailer.
U-haul sounds like the ticket for you since it can be a one-way haul.(if you wanted)

Has to be round trip anyway, I have to return the trailer to the person I'm borrowing that from. I'll talk to uhaul and see what they say. Thanks!

The only places I know of to rent a(box truck)truck is from U-Haul,penske or Ryder.There not all one ways, ask to rent a local truck. But you will have to pay mileage and by the day.You might check the local car rental places maybe you can get a one ton van or truck with a bumper hitch.
Another thought: If your friend has a trailer then he has a truck too right? rent him a car while you barrow his rig.
Trucker