PocketMine-MP is server software made through community reverse-engineering efforts, and, as such, it doesn't have 100% of the game's functionality available. Mobs are one such missing feature at this time, but they can be added with plugins. Others, such as command blocks, redstone, and loading PocketMine worlds, are not currently possible to add.
Adding mobs with PureEntitiesX
The most common plugin to add mobs to a PocketMine server is PureEntitiesX. Before installing it, be sure to check the supported API versions to ensure that it matches up with your server version. To install PureEntitiesX, follow the steps below.
Download PureEntitiesX from here.
Hover over "Customize" in Prisma, click "File Manager", then open the
pluginsfolder by double clicking it.
Drag and drop the
PureEntitiesX.pharfile from your computer into Prisma to upload it to your server.
Once the upload is complete, restart your server to load PureEntities on your server.
What about Bedrock?
The Bedrock Alpha Server is provided directly by Mojang and should carry over all features that you'd see in single-player Bedrock Edition.