As highlighted in the comments above, jQuery by itself only supports two types of easing functions, linear and swing. This is outlined here in the jQuery documentation, which also points out that you can add additional easing functions by including jQuery UI in your project, this could be done by including the jQuery UI CDN in addition to the standard jQuery library:
<script src="https://code.jquery.com/ui/1.13.2/jquery-ui.min.js" integrity="sha256-lSjKY0/srUM9BE3dPm+c4fBo1dky2v27Gdjm2uoZaL0=" crossorigin="anonymous"></script>
Once you've included jQuery UI, you can use the easing functions it provides. There are a number of easing functions you can use, and you can find a list of each easing function and its implementation by logging the $.easing object. A list is also included in the docs:
linear, swing, easeInQuad, easeOutQuad, easeInOutQuad, easeInCubic, easeOutCubic, easeInOutCubic, easeInQuart, easeOutQuart, easeInOutQuart, easeInQuint, easeOutQuint, easeInOutQuint, easeInExpo, easeOutExpo, easeInOutExpo, easeInSine, easeOutSine, easeInOutSine, easeInCirc, easeOutCirc, easeInOutCirc, easeInElastic, easeOutElastic, easeInOutElastic, easeInBack, easeOutBack, easeInOutBack, easeInBounce, easeOutBounce, easeInOutBounce
You can then use one of these easing functions for your counter, eg:
<div class="counter" data-TargetNum="5" data-Speed="5000" data-Easing="easeOutQuad"></div>
See example below:
<!-- Include jQuery -->
<script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.3.1/jquery.min.js"></script>
<!-- Include jQueryUI -->
<script src="https://code.jquery.com/ui/1.13.2/jquery-ui.min.js" integrity="sha256-lSjKY0/srUM9BE3dPm+c4fBo1dky2v27Gdjm2uoZaL0=" crossorigin="anonymous"></script>
<!-- Include js-animated-counter (note: you should download and include the file locally rather than using this link) -->
<script src="https://pablog1.github.io/js-animated-counter/multi-animated-counter.js"></script>
<div id="counters_1">
<h3>Linear:</h3>
<div class="counter" data-TargetNum="5" data-Speed="5000"
data-Easing="linear"></div>
<h3>Swing:</h3>
<div class="counter" data-TargetNum="5" data-Speed="5000"
data-Easing="swing"></div>
<h3>easeOutQuad:</h3>
<div class="counter" data-TargetNum="5" data-Speed="5000"
data-Easing="easeOutQuad"></div>
</div>
Note that you don't need to include the entire jQuery UI library in your project and can instead select just the Effects core component for just the easing functions, which you can do on their download page here.