Right side is a reference to pics/vids of the real-life Grass Wonder lying down and munching grass. See twitter #1375092656829829121.
Left side... I dunno, just Golshi being Golshi, I guess.
Anyhow, the real-life Grass Wonder was (IIRC) considered to have a "decent" and "obedient" temperament during his racing days (read: boring for horse fan memeing purposes) , but after retirement he started picking up quite a few idiosyncratic behaviors (read: not so boring now), including:
Lying down and munching grass and dandelions with his head resting on the ground or on its side, like what Uma GW is doing here. While being filmed.
Daintily picking out dandelions on the grass and eating them. While being filmed. While the Uma GW's seiyuu is watching him in said filming.
'Chewing' on a dandelion stalk in his mouth. The same dandelion stalk. Repeatedly. For quite a while. While being filmed.
Falling asleep while being filmed. And drooling from the edges of his mouth. Sometimes while chewing.
...Yeah, he's really enjoying his retirement.
Umamusume's Grass Wonder is sometimes drawn doing some of the above, as a reference to the real life Grass Wonder's 'antics'. They usually involve dandelions, because the vid with her seiyuu watching the RL GW munching on dandelions kinda went viral slightly.
(There were also a few "fake news" vids and pics mixed in of different horses munching on grass and dandelions, that superficially have the same color as GW. Likely people karma/view-farming.)
