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I'm trying to come up with ways as to how a ship could travel places with a very low number of people (like 4 crewmen). One of the ways I like for how to do this is to have large sea creatures harnessed and used like horses. How big would a sea creature need to be to be able to tow a cog about 25 meters in length over long distances in an ocean? (Or how many creatures?) Any ideas about possible domestication and feeding are welcome. It is a fantasy world, but I don't want the answer to be some sort of magic.

I think the most important property is a suitable body for the harness connecting the cog to the tug (?) animals, or swim team -- sounds like a bad pun.

The cog will need to be a little extra buoyant in the bow to oppose the downward force generated by the swimming monsters.

The dermis of most aquatic creatures is not going to accommodate a hemp rope looped over their bodies since it would rub and wear too much. And, fish don't have shoulders so it will be a challenge to make a yoke that will efficiently transfer the fishes thrust to the ship.

For those two reasons, I can see giant sea turtles as being draft animals. The yoke could be mounted to their shell without causing them pain or distress. Then, all the yokes could connect to a single tow cable.

If turtles are not attractive, then I could imagine a yoke that conformed to the fishes face, and the fish would swim into it to pull the ship forward. Alternatively, these yokes could be mounted anywhere on the ships hull. The inner surface would be a soft felt that protected the piscine dermis from rubbing -- like moleskin. The body would wriggle and friggle to produce the thrust.

I think as long as the creatures are large enough to overcome the friction between the water and the hull, and the waves and the hull, then they'll be able to accelerate the ship to its best speed.

Another thought, giant remora-like fish attach themselves to bottom of the copper plated hull and move the ships around, except these actually swim rather than try to ride for free.

Now, ideas about domestication and such until a specific species is identified.

One problem you'll have with believability in this is that most sea creatures are built for efficiency -- dolphins, swordfish, and barracuda for speed, sharks for energy conservation and feeding efficiency, whales for efficient filter feeding or prodigious deep diving -- not for pulling force.

That said, trained dolphins are on record and film pulling small boats. Generally this is an adult dolphin, 6-8 feet long, pulling a tiny dinghy suitable for 2-3 adults at most, and they don't do it for long and seemingly can't pull it much faster than a human can row. Further, this activity doesn't scale well; the drag of a 25 meter cog is hundreds of times that of a 3-5 meter dinghy (not to mention it's far heavier, so takes more energy to accelerate).

A useful scaling factor is to examine the sail area required for the sizes of the vessels -- a small skiff needs a few square meters; that cog needs a few hundred square meters to get anywhere in a reasonable time. That means it would need the power of a few hundred trained dolphins -- and a pod that size, if free to come and go in order to feed themselves, would eat everything they can catch over a hundred square miles as they passed.

Size isn't really the number you need. It's power, how much water can the creature shift around it's body.

By this measure, you need an Orca. 1 would be enough to get you moving, if you have to fight wind, you will need a few.

A ship being pulled from below the mounting point is self defeating. Your ship would have to be pulled by some sort of keel hanging below the bow of the ship. This is obviously impractical.

Instead, the Orca can have a rigid harness that pushes the ship. The ship would still use a rudder to steer. This also means the ship is in control of the direction traveled, not the Orca. The Orca is also protected by the massive ship in front of it. The Orca also need to fear being crushed by the ship if it stops or changes direction suddenly (possibly for shallow water).

If the ship has sails, the Orca can rest and be dragged along by the ship instead, rather than being crushed by it.

And finally, the Orca pushing the ship will be almost uneffected by rough seas, however towing the ship would result in the bow of the ship lifting up, and yanking the poor Orca around. With the amount of weight of a ship and an Orca, I expect the ship to break in half.