Suppose that there was a tribe of nomadic mermaids whose lives were primarily centered around following pods of dolphins to eat.
They live in the equivalent of the frigid antarctic seas. As they possess both lungs and gills, they can surface above the water or swim in the watery depths for hours on end without having to worry about drowning or suffocation. Their strong claws can easily chip through ice in a manner of minutes.
This is where the ice plays in.
In this setting, the mermaid cut floe or small icebergs away from the land and hollow holes in their sides. That is where they rest and temporarily store their food. I haven't made up a name for them yet, so I'll just call them trailers for the purposes of the question. Through the use of whale sinews, the trailers are tied to the titular pack animals, which transport the trailers to where the mermaids want them to go.
I'd rather not have them ride the currents, because they wouldn't be able to make sails, and also because they wouldn't be able to float away from assailants very quickly.
What real-life marine creature would be best suited as a pack animal?
Keep in mind that their technology is medieval at best.