Scaling from a walrus would suggest 500-700 lb, but I'm not sure if that's the body shape or not.
The first line of flavor needs to be "used as pack animals..."
Anyone wanna buy a used pack animal?
Updated.
Since they are used as pack animals, we ought to include carrying capacity (and possibly a training section).
I had 229.5 lb as the upper end of Medium; don't you always round down in D&D?Yes, since it says they're friendly and easily domesticated we should have their training DCs as regular Animals, without the usual +5 for Magical Beasts.
As for carrying capacity, they work out up to 114 lbs for a light load, 115 to 230 lbs for Medium, and 231 to 345 lbs for a heavy load.
I had 229.5 lb as the upper end of Medium; don't you always round down in D&D?
I'm ok with stating they can be trained as animals. I guess we should come up with a price. They should be worth less than a hippogriff, I think, since they can't fly. Young are worth 1000 gp and training costs 750 gp?
500 gp training was another option I had, so that's fine too.
Ahh, I thought I'd gotten 230 for 1.5*154, but I think I got 229.5 for the upper end of medium load, rounded down, and took 230 for heavy. Might as well use your numbers and just round up on the medium load.