I didn't even think about Leatherhead being a mutation that Yoshi witnessed. I was thinking he was a mutation that happened after Yoshi's/Splinter's - depending on which direction gets followed.
Maybe they were mutated in the sewer/basement with Yoshi, because when the reptile minds developed humanoid sapience, they had the duckling phenomenon and attached themselves to the nearest thing around. Which the Utroms determined to be Yoshi/Splinter since they wanted him to have companions. Perhaps they would have learned this from mutating Leatherhead earlier than the turtles, hoping he'd be the companion, but discovering he attached himself to Dr. Xeinos.
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