Ah, you're right about the other phantasms. But I think it is also to help notify other knowledgable good-aligned creatures.
Treasure Type U said:10-80 gems: 90%
5-30 jewelry: 80%
Maps or Magic Items: 1 of each magic excluding potions & scrolls: 70%
Treasure: No coins; triple goods (gems and jewelry only); double items (no potions or scrolls)?
Advancement: 5-16 HD (Large)?
A typical phantasm stands 8 feet high at the shoulder, weighs x pounds, and has a wingspan of 20 feet.
Flyby Attack and Iron Will seem good feat picks, the same as a Pegasus.
Looks good, although triple items is tempting.
Shouldn't that be 6-15 HD? I'd also fancy having them advance to Huge somewhere along the line. Perhaps 6-9 HD (Large); 10-15 HD (Huge).
8 feet at the shoulder is pretty big. A very large horse is about 6 feet and 2000 pounds, and I'm thinking the same 2000 pounds would suit the Phantasm, on the presumption it's got a lightweight body and very long legs. Which doesn't seem that unreasonable, since it's a flier.
Wingspan could be more than that though - 25 feet? 30 feet?
once every 111 years, the phantasm may plane shift itself from the Ethereal Plane to the Prime Material Plane. It must shift back to the Ethereal Plane within one year, or the phantasm permanently loses 1 hp per day over a year that it stays, until the creature dies.