The meaning of flash in the way I use it is pre-drawn tattoo art offered by artists. This is in contrast to custom tattoos. In my community, there is a much shorter wait to get flash than a booking for a custom piece. The cost is usually less too. An artist might offer flash as a way to pick up extra clients without the effort of custom work because they often don't allow too much customization, and often specify allowable size ranges and areas where they are willing to tattoo. It's easier to estimate the duration of the appointment so they can fit more appointments in. It might be a tattoo that appeals to them personally or a technical challenge they want to add to their portfolio. Some artists will do the designs only once, others will repeat.

The cat in my tattoo is in place of the demon in the original art, not the horse. The horse is absent entirely. I don't know why the artist chose to do that. I didn't ask.