The Meaning of Life, the Universe, and Everything.
If the platypus was added it would be a passive but defensive mob just like the pufferfish. They would spawn primarily in rivers, and rarely in the various water holes around your world. If you get close to the back of them they would use the venomous barb on their foot to sting you doing half a heart of health and giving you a 5 second poison effect. They would drop 0-1 poison barb upon death which you could use to craft the new poison dart item. An Easter egg also exists, as if you name a platypus perry it would have a top hat on as a reference to perry the platypus
We already have poison tipped arrows, so darts might be a hard sell. This otherwise just sounds like an immobile small-pool pufferfish, which I'm not against but also not really amazed by. At least it would add another hazardous animal mob to the overworld daytime. It feels a bit too safe out there now so long as you avoid the massive 100-block falls into huge caves.