We use mosquito fish in ours...they look like little colorless guppies and survive very easily without any care where I live. They're not at all cute like goldfish though!
One word about cinderblocks...I'd recommend not using those at all unless you cure them with vinegar. They leach toxins that kill fish and I'd think are probably just as bad for a horse. If you google building a cement pond, the last step should be curing the cement to make it safe and it'll tell you how to cure them.
Old drift wood is a really good choice you could use though. The wood is a good material for pond bacteria to grow in to help clean up fish wastes.