Argys are a great steed. They can carry small mounts, including Dire wolves and Sabertooths. Knocking them out is pretty simple, using a longneck rifle. Two shots can knock em out pretty nicely. Feed them with meat. Their pretty slow, but strong. Good tame!!
An argentavis can't fit through a ceiling with one tile missing, but a pteranadon or lymantria can. Build a trap with one corner ceiling missing, aggro the argentavis, fly in one side and out the missing ceiling tile, and land. Dismount, run around, and close the entry door. Tame safely at your leisure.