I came across this sentence in a dictionary and I think this sentence is wrong. Because I would write this sentence as, "The sheep were loaded into trucks".
Please help me.
Onto can be used, as well. It`s rather an informal usage of this preposition, especially in Am.E., but I agree with you that "into" is more appropriate in your sentence.
Here is what I`ve found:
I can very vividly remember how exciting it was in Magdalena during "Shipping Season" when all the cattle and sheep were driven in from the ranches to the west and loaded on the trains. There were thousands of sheep and cattle shipped out every October and November from the time we were little kids until we were in High School. Somewhere in those years, early to late 50's things began to change as now much of the livestock "was being loaded onto trucks" and hauled off from the stockyards. Then the trucks started picking up livestock at the ranches which meant the ranchers no longer had to drive their cattle and sheep to Magdalena. Someone told me once that the last major shipment of livestock out of Magdalena by rail was in the early 60's. The trains stopped coming to Magdalena altogether in about 1970 and the rails were pulled up in 1971.[source :http://www.jocastilloart.com/Who.html]
Here's what I think about the usage of these prepositions: People (and things) ride in private transportation and on mass transit. There's no rule about it in English; it's all idiomatic.
This morning I got on a bus, rode it to the train station, got on a train, went to the airport and got on a plane. I might ride in a taxicab, in a car or in a motorhome (caravan). As for trucks, if I'm in the driver's compartment, I'm in the truck; everything in the cargo area is on the truck.