@Martimer

The chances are pretty slim (1.8%), and low-level pets are much less likely to find legendaries, so you're extremely lucky! Congrats! As to whether you should keep or sell it, that's up to you. Regular Xeldron (the one you have) tends to go for 1000 PC on the market. That would give you access to two different house upgrades already (Small European Rowhouse and New Luck Bungalow). That would give you a nice leg up in the game, and you can always find or buy another Xeldron later.

wow, congratulations! let's hope this sets your tone of luck for the rest of the game

Impressive!

Xeldrons are a great find, congrats! They take longer to hatch being a legendary, but they are worth it, especially when you manage to get them levelled up a bit.