Krishnath, that would be cool. (I'm trying to think which fantasy books have had evil black swans in them. I definitely remember the Fionavar Tapestry books here.)
Inspired by a discussion on the Usenet D&D newsgroup, I did a websearch for stats for Giant Swans in D&D. (The reference was to the aerial mounts used by the good guys in the 80s arcade game Joust. Think of that, and you have the hook for this creature. They might be a good choice for more 'civilized' elves or lowland humans.)
I found one definition for Giant Swans on the Middle Earth/D&D page, but I didn't like it entirely, so I tweaked it a bit. Comments? I used the CC calculator to get the CR; it makes sense that it would be lower than for eagles or owls, I think, because of the lack of special ability and lower-damage attacks.
I do wonder whether the wings should do less damage and the beak more? (I used the slam value for large magical beasts in the How to Create a Monster article.)
Giant Swan
Large Magical Beast
HD 4d10+8 (30 hp)
Init +3 (Dex)
Spd 10ft, fly 70ft (average), swim 30ft
AC 14 (-1 size, +2 Dex, +3 natural)
Attacks: slam +7 melee, 2 wings +2 melee
Damage: slam 1d4+4, wings 1d4+2
Face/Reach: 5'x5'/5'
Special Qualities: none
Saves Fort +5, Ref +7, Will +3
Str 18, Dex 15, Con 14, Int 10, Wis 14, Cha 10.
Skills: Knowledge (nature) +6, Listen +8, Sense Motive +8, Spot +8, Wilderness Lore +4
Feats: Endurance
Climate/Terrain:
Organization: solitairy, pair, or flock (10-30)
Challenge Rating: 2
Treasure: none
Alignment: usually Neutral
Advancement: 5-8 HD (huge); 9-12 HD (gargantuan)
If encountered as a pair, they would be a mated pair and if nesting are very territorial. Flocks are typically encountered when migrating, in which case there will be no eggs.
In combat, giant swans slam with their beaks and buffet opponents with their wings.
11' tall with a wingspan of 21 feet
Speak Common and Auran
Training, Carry Capacity: same as Giant Eagle or Owl
Highly prized for their dependability and endurance as aerial mounts.