[Vrul]

semlar gave you the best way but if you still wanted to see your original method shrunk a bit for educational purposes:

/run local f,p=IsFlyableArea(),"MOUNT"
for i=1,GetNumCompanions(p) do 
    local _,v=GetCompanionInfo(p,i)
    if f and v=="Obsidian Nightwing" or v=="Vicious Skeletal Warhorse" then 
        CallCompanion(p,i) 
    end 
end