So I had the vet out today for Onyx. Vet is puzzled, so he took some pictures and is going to consult with the senior veterinarian at his practice. He also mentioned doing research.
Some good things: He doesn't think its white line disease, nor a selenium deficiency. He said the number one thing he can think of it a toxic plant in our area that is quite common, and if eaten in large quantities can cause issues. However, he said that it is the wrong time for that, since those plants have not yet bloomed.
He said it could be diet related, but he doubts it as Onyx has not had any major changes, and he estimates if it was diet it would have been a major change in the past few weeks (given where the lines on the hoof are). He did not completely rule that out.
He did say not to worry too much. I was going to go back to my hometown this weekend to celebrate Easter with my family, and he said it would still be alright, as he doesn't think it is going to get any worse over the weekend. Onyx is still his normal horsey self, no lethargy. My friend, who owns the other horse on the property, agreed to keep an eye on him for me.
Vet did ask about founder or abscess history. As far as I know, Onyx has had neither. Since I have had him (almost a year) he is on full pasture almost 24/7. Since it is spring, however, we have slowly reintroduced him to grass in 2,4,6,8 hour increments over a period of 2 weeks. No other signs of founder or discomfort are present.
Anybody got any opinions on this? It sure is weird!