I tried the following and I still can't get chickens to fly. Am I doing something wrong?

Attempt #1:

{
    "Chicken": {
        "abilities": ["fly"],
    }
}

Attempt #2:

{
    "minecraft:chicken": {
        "abilities": ["fly"],
    }
}

Attempt #3:

{
    "minecraft:chicken": {
        "abilities": "fly",
    }
}

Attempt #4:

{
    "Chicken": {
        "abilities": "fly",
    }
}

I've run out of ideas at this point. :( I can morph but no flight. Tried making HP into 1 as well, nothing on that either. Am I doing something wrong?

In all of these attempts you posted, you have invalid JSON. As stated in the Wiki:

IMPORTANT: when editing JSON, PLEASE, make sure to check whether modified file has a valid JSON code using a JSON linter, such as this. This will save both mine and your time.

The correct JSON would be for 1.11.2+ (as you can see, there is no extra comma on the end of abilities):

{
    "minecraft:chicken": {
        "abilities": ["fly"]
    }
}

and for 1.10.2:

{
    "Chicken": {
        "abilities": ["fly"]
    }
}

My apologies, that was a copy paste error as I didn't include the longer portion which also included cows, pigs etc flying too (as I got lazy looking for only chickens). I actually copy pasted the wolf with 40 HP etc from the guide and it still had wolves with 4 hearts. I'll double check once I'm home regarding the chicken to ensure I got the json code correct and let you know. Should the wolf and bat thing on the guide definitely work for 1.12.2 with "minecraft:wolf" and "minecraft:bat" instead of "Wolf" and "Bat"?

EDIT: Figured it out! I had thought the HP would change for the mob by default, not only once you morph into it. My bad. :D I'll close this down as user error.

👍