A player in the new campaign I am DMing wants their character, a halfling wizard, to ride around on a small bird, going for a Sir Didymus from Labyrinth feel. He suggested this could be his familiar, though he was very clear he doesn't expect this give him any special advantages (increased speed, flight etc.) beyond those a familiar normally grants. I liked the idea, so I came up with the following creature which would be added to the list of animal forms for the find familiar spell:
Bococho (riding familiar)
Small fey, unaligned
HP 3 (1d6)
Speed 25 ft. (cannot fly)
STR 7 (-2) DEX 15 (+2) CON 10 (+0)
INT 2 (-4) WIS 14 (+2) CHA 7 (-2)
Skills: Perception +4
Senses: Passive Perception 14
Magical Mount. A bococho can carry its master (including their belongings), as long as they are of size Small or smaller. It won’t carry anything else.
Peck. Melee Weapon Attack: +4 to hit, reach 5 ft., one target. Hit: 4 (1d4 + 2) piercing damage.
It's based on the eagle's stats (since that's a Small-sized bird which is still CR 0), but with no flying speed (it's flightless), a better walking speed (matching a halfling's normal speed), no Keen Sight trait, and the new Magical Mount trait allowing it to carry its master and for simplicity's sake to magically ignore how much gear they're carrying. (For reference, the slightly increased Strength of 7 means it can carry up to 105 pounds, which seems pretty reasonable for carrying a halfling wizard, who will only weigh around 40 pounds plus gear.) It retains the eagle's hit points and (slightly tweaked for theme) attack.
Is this balanced? I'm pretty sure that the hp and attack won't be an issue, since it still has a tiny number of hp and it can't attack in normal circumstances anyway, but have I made it too weak removing it's ability to fly at all (rather than just specifying it cannot fly while being ridden, say) and removing it's sharp sight feature (advantage on Wisdom (Perception) checks involving sight)? Are there any drawbacks or advantages to this familiar that I haven't considered?