I think your the first person to ask about Catfish for a pond.
There are a few species of Catfish that you can keep in a pond but you have to remember that companions must be chosen carefully!
Channel Catfish,
Ictalurus punctatus, these large catfish can reach upto 48", often sold in pond sections as an Albino form. They have been successfully kept in large Koi ponds.
Bullheads, there are two species of Bullhead -
Ameiurus melas or Black Bullhead and
A.nebulosus or Brown Bullhead, these can be kept in tanks or pond in coldwater or low tropical temperatures. They do best in the aquarium as in a pond they will hardly ever be seen. These 30cm/12" catfish can survive in water with a very low oxygen content and are often shipped over in the hundreds as tiny 1-2" specimens.
Wels Catfish!
Silurus glanis now this is just for mentioning not keeping! The Wels Catfish is one of the largest FW fish on earth, fish have been caught at over 16' all that I can think of that compares to this are the
Arapaima or Gaint
Pangasius Catfish of Asia, the large South American Catfish that alot of people think should not be kept in the home aquaria are minatures compared to these! If anyone ever wanted to keep these fish are pond of 40,000+ gallons for just the one fish is needed but you also have to consider your safety......