Since there's only one texture for the goat's ears, the UVs should be mirrored on one side.
Otherwise any kind of detailed texture will look wrong as in my resource pack image (attached below).
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May relate to MC-130026.
Can confirm in 21w15a.

Does this also apply to their horns and/or legs? I can't tell
@Orbic
Each leg has a unique texture.
Both horns use the same UVs and are not mirrored.
Arguably they should be, but it is less of a problem that the non-mirrored ears.
Can confirm in 21w16a.

Can confirm for 21w17a
Can confirm in 21w14a.