Any Mr. Ed tricks would be a no go in any games I run. Animal companion or not, he can't write what he wants in the dirt. I don't care how smart he is. ;)
Maybe RAW it's allowed, but I wouldn't run it as such. Or ok if you really want to argue that it can write in the dirt with it's hooves fine... but your animal companion has the worst penmanship this side of the Mississippi. You've heard of chicken scratch, this will be horse scratch. ;)
And if you still want to argue that it can write long elaborate letters, than congratulations, you've discovered a way to min/max even animal companion linguistics skill. ;)