IIRC the DP font has had this problem for several people, and Arial might sometimes as well, although I haven't seen it in recent versions of Essentials so it may have been fixed. Are you using an up-to-date version?
If you're seeing Arial then presumably you don't have the included Pokémon fonts installed. Try going into the Fonts folder of your Essentials project and manually installing one or more of those fonts, and see if changing to those fonts in the game itself fixes the problem (assuming an Essentials update also doesn't fix it... although you'd probably want to use the included fonts anyway, as Arial doesn't fit comfortably in some screens/text boxes to begin with. Things like move/ability descriptions and so on are sized to fit the Pokémon fonts, and even if the Arial text has proper spaces it still tends to run off the side of the screen in places.)