Ancient Golden Crystals go consistently for 3.5 million gold on Alexina. Ancient Magic Crystals go for 500k. Shining Metal Shards are like 2.5 million gold.
Squires are essentially a minigame where you send Squires on missions that will help them become stronger, rewarding you in the process with Baltane seals in Basic, Intermediate, and Advanced tiers. The strong the Squires become, the more rewarding missions they can take.
They take time to max out, but once all those 450 stats are reached, you essentially have to do maintenance every other week or so, but you can essentially just lay back while praying for RNG to reward you.
Squire medals can also be fragmented into Divine Magic Powder, which sells for quite a bit, and is a guaranteed drop in Squire specific missions.
I think it's after like 5 hours of being alive that monsters have a chance to turn ancient, and it's just a chance so its probably coincidental that 4 of them turned ancient, and if it was a lowly populated channel, or at night time, I wouldn't doubt that they had time to turn ancient before you found them.