Some creatures have a fly speed. Some creatures with a fly speed also have the ability to hover specified.
The PHB, in Chapter 9: Combat, Movement and Position, Flying Movement says:
Flying creatures enjoy many benefits of mobility, but they must also deal with the danger of falling. If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic, such as by the fly spell.
The conditions in this paragraph that would result in the creature falling have to do with having movement involuntarily removed or restricted. That might suggest that if you don't have "hover" in your flying speed, you can't hover.
But there is a difference between being knocked unconscious, or frozen, or webbed, or otherwise rendered unable to move, and choosing to hover, by flapping your wings, or standing on your jets, or whatever.
So, can flying creatures choose to hover, even if they don't have hover in their flying speed?