Gibbon has an ID of 120, Pheasants/TurtleDove are 104.
Changing monster.dat won't do much, there's nothing really in it used to generate the ID. The TypeID is the closest thing, but no cigar.
The mesh ID is actually sent to the client for every monster, and unless you intercept every packet of that type and modify the mesh otherwise, you'll need to change all the textures and meshes. The texture alone will just paint over a Pheasant mesh with the Gibbon texture, which will look rather odd. There is also the problem that changine a mesh will change it for all monsters of that type (eg, MeteorDove, TreasureBird etc will be affected).
So, there are 3 files you need to modify. 3dtexture.ini, 3dobj.ini, 3dmotion.ini.
These take an ID and reference the files. For monsters, you multiply their ID by 1million to obtain their texture ID. Search for that ID within the files, and modify the file it references to.
For example, in 3dtexture.ini, find Pheasant texture.
104000000=c3/texture/104000000.dds
Modify to this:
104000000=c3/texture/120000000.dds
Do the same in 3dmotion and 3dobj. 3dmotion has a long list for each mesh, and you'll need to change every filename, quite tedious. You can never guarantee that the motions will be correct either.
Mesh 104N is used for Pheasants, they were modified in Co2, hence the N for new. There's some other monsters the same. Their ID remains 104, but the filename/path is 104N.
It's a bit of a waste of time, I wouldn't bother tbh.