Your environment could be extremely hostile to smaller creatures, resulting in various strategies to become large. You'll have a lot of creatures which just have many offspring which grow extremely rapidly in the hopes that some survive, but other options are available.

Your multi-headed species engages in a type of symbiosis, where its offspring develop on its body with a shared circulatory and nervous system. It would have multiple heads which could watch in several directions alerting it to potential threats. It will always have a range of head sizes from a single large one ranging down to tiny developing heads. The larger heads would act more independently, trying to eat their own food and potentially disagreeing with the others. As a head reaches maturity it would develop a longer more muscular neck (actually the rest of its body developing) before gaining legs and separating from its parent. A newly separated creature would only have one head for a time before it developed offspring of its own.

Your environment could be extremely hostile to smaller creatures, resulting in various strategies to become large. You'll have a lot of creatures which have many offspring which grow extremely rapidly in the hopes that some survive, but other options are available.

Your multi-headed species engages in a type of symbiosis, where its offspring develop on its body with a shared circulatory and nervous system. It would have multiple heads which could watch in several directions alerting it to potential threats. It will always have a range of head sizes from a single large one ranging down to tiny developing heads. The larger heads would act more independently, trying to eat their own food and potentially disagreeing with the others. As a head reaches maturity it would develop a longer more muscular neck (actually the rest of its body developing) before gaining legs and separating from its parent. A newly separated creature would only have one head for a time before it developed offspring of its own.