My ultimate problem is solved. I will explain with the crayon that I needed to use in order to understand what I was doing wrong. I knew it had to be something stupid since I couldn't find it anywhere online (I'm not a real modder, just editing stuff and getting into trouble).
OpenIV doesn't have support for RDR2 so you have us it to extract files to an XML file. Open in notepad++, edit, save as with the .ymt extension (I kinda already knew this part but the game was crashing when I spawned in the file). In the .ymt file, it has "<UNK_TYPE_....". at the very top and bottom. "Type" has to be changed to member or, as what I did, delete the UNK part and just have the hash code (<0x4BD672F>). Having "type" is the part that was causing the game to crash when I spawned it in. All is well and I have a new horsey.