"Moo!" was the first sound Horsie ever heard, so he kinda thought it was the natural thing to say...
from the StickHorsie™ origin story (yes, Horsie is from the future
and yes, there are several Horsie stories available, by several writers even!):
They went to work. Merging replicant-DRNA and cobalt-iridium alloy mesh, connecting servo motors and command chips, tightening microbolts, using and adding all kinds of things that were invented recently, but never used. And six hours of hard work later (and enjoying every minute of it) Stick Horsie 1.0 was ready for its first trial run.
"Everything checks out.... let's see if it has some mobility!" Erjeon pressed a touchpad and bent over towards the small microphone on the control panel. "HORSIE, MOVE YOUR RIGHT LEG! MOVE YOUR LEG!"
Nothing happened.
Yet Kiw'lee broke out in laughter. "It will help if you actually plug it in, you know. The wire, yes?" Erjeon grinned sheepishly, fumbled with a couple of plugs and pressed the pad again. "HORSIE, MO-"
A loud explosion outside rocked the building on its foundations. "A.T.A! A-Tech Alert! A.T.A.!" a screen flashed in angry red letters. "Oh great," grumbled Erjeon. "Here we go again!" And with a sigh and a final look at Horsie, Kiw'lee and Erjeon left the laboratory. It was to be quite an ordinary day after all.
They should have stayed a little, maybe. For just a couple of seconds later, Horsie stirred into life. It moved its head, looked about a bit, moved its legs and repeated the only command it had ever heard.
"Moo?" it said, questioningly. "Moo?"
Later, during the flight from the lab, Horsie's voice processor gets damaged and he reverts to projecting word balloons, as seen in the Stargate-SG1 story where Horsie unexpectedly drops in...
"Sir! I think it’s trying to communicate!"
Siler drew their attention back to the figure, which indeed looked as if someone, in a moment of sheer inspiration, had taken a blue marker and drawn a rather cheerful stick horse on the closed iris. Above the creature’s head, something that looked suspiciously like a blue outlined conversation bubble had appeared, and letters were starting to form on the milky white surface in the center.
The General frowned at the new development. "What’s it saying?" he asked.
"Looks like ‘Moo!’" said Colonel O’Neill, leaning forward and squinting through the thick glass that shielded the control room.
“Moo?” questioned Daniel, “Why would a horse be saying moo?”
