(Posted in the SEA repo as well)

Both Pehkui and Size Entity Attributes require mixing into HorseBaseEntity to modify a passenger's offsets. Both mods require it for the same reason, but since they both use @ModifyConstant with the same priority, they force only one of the two mods to succeed, which causes problems.

I will submit a PR soon to disable this mod's Mixin for HorseBaseEntity if it detects the other mod, and I will create a small bridge mod to sit between the two to provide compatibility.

Since SEA has been discontinued and the author now recommends using Pehkui instead, this is effectively resolved.