What I want to do is use the execute command to randomly pick a custom named mob or entity. I already have this command:
execute @r[type=Boat] ~ ~ ~ setblock ~ ~-2 ~ redstone_block
which I am using to place a redstone block and select one of five categories, but then if I want to use the same command on a sheep, for example, I want it to select from the ones I summon and not randomly find one in the wild (which I figure that's what it's doing, because it was saying that the command was executed properly). The problem is that I have no idea how to go about making that work. I've tried a couple of things, but it either came back with an error or executed it on a random mob in the wild.