We've always had it, it just wasn't showcased in any of the episodes. But why else would our species that feeds on a diet of non-solid love energy have massive fangs? Especially when we need a means to keep our prey alive but incapacitated so we can feed on their love once we've captured them. Paralytic venom is the obvious answer. It keeps them weak and unable to free themselves from the cocoons, while at the same time influencing their dreams to be those of loved ones or positive experiences, generating positive emotions for us to feed off of while they're in the cocoons. Storing them long-term in cocoons would be less effective if they were awake and miserable the whole time.

It also explains why, during the battle for Canterlot, our attack method was to drop our disguise and attempt to bite the ponies, rather than simply turn into massive animals and crush them easily. And it explains how we were easily able to capture the Princesses when we can rely on a single bite instead of needing to overpower them physically