[2019] With POF nerf coming, scale the animals drop rate in a reasonable way.

The purpose of the drop rates was so that the primary sources of animals coming into the game were breeding and then player trades, which is what occurred. Nothing to do with flooding the market. We've subsequently added the farmhand who obtains animals for you, which helps Ironmen.

There is no XP advantage between breeds. It's harder for Ironmen to compete the breeding log, true, but the only reward for doing so is the achievement (maybe that's a requirement for a cosmetic too? - title - I forget). I think the only place you lose out is with certain animal breeds' harvest materials, and in my experience that pain is only really felt with chinchompas.

Not that I'm opposed to varying drop rates for chins, I'm just not convinced by this reasoning. It's perhaps minimal, but there is already some inherent scaling in there via core Hunter mechanics (higher levels increase success chance for things below your level).

Which is primarily now a cosmetic, no (since the majority of its benefits were separated and/or are shared by the max cape)?

Right, but that's why they are tradeable. It adds value to lower-level Hunter for players at that level catching them, and selling them to higher-level Hunters/players who don't want to waste time hunting well below their level. But even then, the value of obtaining all breeds is relatively small. You can get the majority of value out of POF with just one breeding pair of any colour chinchompas (something the Trapper can help to achieve in time, even for Ironmen, and with not overly awful RNG).