The turtledoves are from Winter Holiday, for which the opening line is: "Dragon (an evil elven wizard) has formed GRINCH (GRoup Intent on Crushing Holidays) with the purpose of disrupting the joys of Winter Holiday." And, as you surmised, they are indeed goofy dove/turtle hybrids.
"[...] ahead are two large shapes moving to intersect the trail. As you close rapidly, the shapes resolve themselves into huge turtles with white-feathered wings. One of the creatures is equipped with a large leather case. [...]
The turtledoves are aggressive and fight until destroyed, but the party may simply choose to fly past the hapless turtledoves, which cannot catch up. [Note: This is because the adventurers have recently ingested magical eggnog with gives them a flight speed of 1024!] The doves will, however, get one attack at -4 as the party flies past.
Turtledoves (2): Int Low; AL N; AC 2 (top), 5 (head, wings, and underside); MV 6, Fl 15 (C); HD 7+7; hp 31 each; THAC0 12; #AT 1; Dmg 2-8; SA bite inflicts serenity, victim must save vs. paralyzation or refrain from hostile actions for 1d4+1 melee rounds; SZ H (6' Diameter); ML 11; XP 1400 each.
Turtledoves are known to fill their lairs with gaudy, hollow glass balls, Dead turtledoves continue to float, thus the shells of these creatures are highly prized as components for potions of flying and other similar magics. One of the turtledoves carries a large leather case, which contains three musical horns made of shiny brass tubing formed into a circular coil."
Also appearing in this adventure are four colliebirddogs (named Spot, Blotch, Stain and Spill), six platemail wearing geese, seven Swimming Swamis (these are NPCs, not monsters), eight Chocolate Milkshades (milking "udderly too many" Holsteins) and a lizardman named "R. S. Claws".
Let me know if you'd like stats for any of these oddballs.