There are quite a few ponies who changed names from one side of the ocean to the other [excluding the name changes which happened in countries where English is not the first language] - you have big brothers, tropical ponies, rainbow curl ponies, cuddles from the magic message ponies, sugarberry from the taf ponies, a lot of the newborn twins and several more.
Its just cultural; what works on one side of the ocean simply does not work on the other, or was judged to not be as effective on the other.
Slugger is a good example. The word Slugger is very American and not necessarily something which would have been understood over here at the time; therefore he became First Base [which perhaps it now has a different connotation
] but at the time would have been more understood to UK shoppers.
So both names are right as Roogna says, it just depends on where you are from which you call them
The US and the UK already has plenty of different names for the same things, so a few ponies changing kind of makes sense too