Well, in the first place, it ought to be easily visible, content to have large shadows, from viewing humans, cast over it but be quick enough to dive to the bottom should the shadow be from a heron or a cat!
Additionally, it should be hardy enough to over-winter safely in the pond, not require too much attention in the way of water quality and, of course, be prepared to breed at a moment's notice.

Being highly visible is a bit of a double-edged sword to the fish: apart from being valued for its beauty, there is the reverse side of the coin - being that much more visible to any predator. The ability to survive in the pond depends on speed, either to grab food ahead of competitors or, again escape from danger.

Speed demands the right shape of body and the right type of caudal fin: a fairly streamlined body, coupled with a crescent or paddle-shaped caudal is just about right. However, when we come up against the many varieties of Fancy Goldfish, for instance, we find that many of these do not fit the required pattern.

An egg-shaped body fringed by long flowing fins hardly makes for a piscine Porsche! Of the varieties of Goldfish just up from the conventional Singletails perhaps the Fantail with its stiffer fins and straighter dorsal outline represents the limit of suitability.

'Overdeveloped' Fancy Goldfish such as Celestials and the Bubble-eyes
(some might say even Orandas and Lionheads) are clearly unsuitable for the outdoor pond.

Maybe the Oranda and Lionhead can see perfectly well from beneath their decorative 'wen' but chasing food (or escaping the net) with inefficient wiggles from their downturned caudal peduncles will soon be lost causes.

Long flowing fins will also be a hindrance in the pond; the slightest fall in water cleanliness will soon create congestion and infection; physical damage from faster swimming companions cannot be ruled out either.

To summarise then, keep the really exotic varieties where they can be seen, looked after and appreciated the best - in the indoor aquarium.

Ideally, a pond fish should occupy the middle and upper layers of the waters; bottom-dwelling species, despite any scavenging benefits they may donate to the pond's well-being will, unless they are a gold variety, be quite invisible from above and only seen when the autumn clean-out of the pond occurs.
However, there is no need to think that the pond's inhabitants are limited to a solitary range of fishes, although the strict number of scientific families or genera may be only in single figures. The selection of body forms and colours within the Goldfish groups is almost indeterminate such are the varieties, and variances available. Remember, pond fish do not have to conform to Show Standards to make themselves popular, many a favourite fish may, in truth, be a bit of a mongrel!

The Orfe (with around three colour variants) is a favourite choice for it is constantly on the move just under the water surface; it may have the habit of leaping out of the water at times, during summer, so owners of small ponds might well find them on the lawn - if the cat doesn't get there first!

Koi make excellent pond fish, especially if you have the room for them to develop to their full potential.

Being bred and developed to be seen from above, this designer-fish must be the ultimate.

Many of the North American fishes can be kept in the pond:
the Red Shiner is an instant thought.

Sunfish are very colourful (some are predatory!) but maybe their colours are again best appreciated in a large aquarium.

Note: Many coldwater species are subject to legislation, requiring a licence for
         their keeping. (see Have You Got a Licence?)

Elsewhere, the Golden Medaka and White Cloud Mountain Minnows and some other 'refugees' from the tropical aquarium can take an annual summer holiday in the outdoor pond: these include American Flagfish, Pencilfish, Paradisefish, Platies, Swordtails, Mollies and Guppies. These latter species usually benefit greatly from a 'stretch' outside and, of course, the green food in the water together with all the live insect food should put them into the breeding (and showing) condition better than anything else.

Select the right type of fish for your pond and it will repay you many times over with pleasure and enjoyment and, who knows, perhaps many more of the same.