The heavy (1/2" diameter) pin is the "clevis pin".
(CORRECTION ! They are typically called "bent handle hitch pins.) (Hitch pins are usually longer, larger diameter and have a large handle.) The thing they are calling a "cotter pin" is actually a "clevis/hitch pin clip" or generically, a "keeper".
There are 2 different styles of clevis pins. One that has a hole drilled through it for the pin clip and one that has a groove cut all the way around the pin for the clip to ride in.
Clevis pins and clevis pin clips have been used for dozens and dozens of years with very few incidents. A clevis pin with a hole for the clip is likely a bit more "robust".
The smallest diameter lynch pin I can find is 3/16" diameter which would require a fairly large hole in a 1/2" diameter clevis pin. More appropriate for a 3/4" hitch pin.
There are several different styles of clevis pin clips. Most of the others I have seen have positive locking but are formed out of thinner wire.
Circle cotter pin
Saftey Pin
