I'm not sure if this was posted anywhere else; I couldn't find it on search. I assume it hasn't because people in the khutit form mod thread don't seem to have seen it anywhere.
Anyway, this is actually incredibly easy to do. Go into your Horizons folder, then from there go to resources-->entities-->characters. You should see files called dragon_u.ent, dragonj_u.ent, and dragona_u.ent among a bunch of others. dragon_u is adult, dragonj_u is hatchling, and dragona_u is ancient.
If you want to have your hatchling appear as an adult, copy dragon_u.ent to the resources_override folder and rename it to dragonj_u.ent. Make sure you only rename/edit this in the override folder otherwise you'll overwrite the original hatchling file and have to do a full scan/replace the file. If you want ancient, copy the dragona_u.ent file instead of the dragon_u.ent file.
That will make any hatchling appear to be adult/ancient. The direct copy should also give you the appropriate emote set.
If you try to go into character creation while this is active, you will notice the camera angles and distances are less than ideal. I played round with that a while and found more suitable parameters.
To fix these, open the .ent file in notepad. For ancients, you will copy and paste this directly on the end of the file, but the line above the ending " } ". For adults, I found some of this already present, so just paste it -over- that.
And that should do it. As a note, the adults/ancients were obviously not meant to fit in the character creation screen, so it helps to set the height all the way down.
float charCreation_distCloseup = 8
float charCreation_distFaceCloseup = 6
float charCreation_faceCloseUpLookAtTension = 15.0
To disable this, just delete/otherwise remove the dragonj_u.ent file from the override folder. This shouldn't cause any problems, but as always I'm not responsible for anything bad that happens to your game/computer/etc.