A horse ran in this race named Bottoms Up. Before they jumped the caller said that Bottoms Up was formerly known as Samaguita.

I had a look at the previous runs for the horse and it was formerly Samaguita.

When can a horses name be changed? I haven't seen it before.

As far as I know a horses name can be changed at any time as long at the proper process is followed with getting new name approved and registered, etc. I have seen it a few times, usually it happens when the horse is purchased by new owners and they don't like the old name.