There exist multiple instances of each boss altar. (It has also been possible for a boss altar to not spawn in the mountains!) Since they'll all be in the same Meadows biome, you're not unlikely to find all three of Eikthyr's.

You'll find stones giving you the map location of the second boss as you explore the Black Forest. These are located in Burial Chambers and in the occasional ruined tower. You'll similarly find stones for future bosses as you explore their biomes.